SUMMARY

      This summary provides a brief overview of information contained elsewhere in this prospectus. Because it is abbreviated, this summary may not contain all of the information that you should consider before investing in the common units. You should read the entire prospectus carefully, including the historical and pro forma financial statements and the notes to those financial statements. The information presented in this prospectus assumes (1) an initial public offering price of $20.00 per common unit and (2) unless otherwise indicated, that the underwriters’ option to purchase additional units is not exercised. You should read “Risk Factors” beginning on page 23 for more information about important risks that you should consider carefully before buying our common units. We include a glossary of some of the terms used in this prospectus as Appendix B.

      References in this prospectus to “Eagle Rock Energy Partners, L.P.,” “we,” “our,” “us” or like terms, when used in a historical context, refer to both Eagle Rock Pipeline, L.P. and its subsidiaries. When used in the present tense or prospectively, those terms refer to Eagle Rock Energy Partners, L.P. and its subsidiaries. References to “Natural Gas Partners” refer to Natural Gas Partners VII, L.P. and Natural Gas Partners VIII, L.P. in the context of any description of our investors, and in other contexts refer to Natural Gas Partners, L.L.C. d/b/a NGP Energy Capital Management, which manages a series of energy investment funds, including Natural Gas Partners VII, L.P. and Natural Gas Partners VIII, L.P. References to the “NGP Investors” refer to Natural Gas Partners and some of our directors and members of our management team.

Eagle Rock Energy Partners, L.P.

      We are a growth-oriented Delaware limited partnership engaged in the business of gathering, compressing, treating, processing, transporting and selling natural gas and fractionating and transporting natural gas liquids, or NGLs. Our assets are strategically located in three significant natural gas producing regions in the Texas Panhandle, southeast Texas and Louisiana. We intend to acquire and construct additional assets and we have an experienced management team dedicated to growing and maximizing the profitability of our assets.

      Our Texas Panhandle operations cover ten counties in Texas and one county in Oklahoma, consisting of our East Panhandle System and our West Panhandle System. The facilities that comprise our East Panhandle System are primarily located in Wheeler, Hemphill and Roberts Counties in the eastern Texas Panhandle and consist of:

      In addition, we recently purchased Midstream Gas Services, L.P., which consists of facilities located in Roberts County within our East Panhandle System. The facilities consist of approximately four miles of natural gas gathering pipelines with associated pipeline compression and an active natural gas processing plant with aggregate capacity of 25 MMcf/d.

      The facilities that comprise our West Panhandle System are primarily located in Moore, Potter, Hutchinson, Carson, Roberts, Gray, Wheeler and Collingsworth Counties in the western Texas Panhandle and consist of:

      Our southeast Texas and Louisiana operations are primarily located in Polk, Tyler, Jasper and Newton Counties, Texas and Vernon Parish, Louisiana. The facilities that comprise our southeast Texas and Louisiana operations consist of:

      We commenced operations in 2002 when certain members of our management team formed Eagle Rock Energy, Inc., an affiliate of our predecessor, to provide midstream services to natural gas producers. Since 2002, we have grown through a combination of organic growth and acquisitions. In connection with the acquisition in 2003 of the Dry Trail plant, a CO ₂ tertiary recovery plant located in the Oklahoma panhandle, members of our management team formed Eagle Rock Holdings, L.P., the successor to Eagle Rock Energy, Inc., to own, operate, acquire and develop complementary midstream energy assets. Eagle Rock Holdings, L.P. has benefited from the equity sponsorship of Natural Gas Partners, one of the largest private equity fund sponsors of companies in the energy sector, which since 2003 has provided us with significant support in pursuing acquisitions, including its equity investment of approximately $191 million to help facilitate our acquisition of the Texas Panhandle Systems and other assets.

Business Strategies

      Our primary business objective is to increase our cash distributions per unit over time. We intend to accomplish this objective by continuing to execute the following business strategies:

Competitive Strengths

      We believe that we are well positioned to execute our business strategies successfully because of the following competitive strengths:

Formation Transactions and Partnership Structure

General

      We are a Delaware limited partnership formed in May 2006 to own and operate the assets that have historically been owned and operated by Eagle Rock Holdings, L.P. and its subsidiaries. In 2002, certain members of our management team formed Eagle Rock Energy, Inc. to provide midstream services to natural gas producers. In connection with the acquisition of the Dry Trail plant in 2003, members of our management team and Natural Gas Partners formed Eagle Rock Holdings, L.P., the successor to Eagle Rock Energy, Inc., to own, operate, acquire and develop complementary midstream energy assets.

      In March 2006, certain private investors, which we refer to as the March 2006 Private Investors, contributed $98.3 million to Eagle Rock Pipeline, L.P., which will become our operating partnership and which we refer to as Eagle Rock Pipeline, in exchange for 5,455,050 common units in Eagle Rock Pipeline.

      In June 2006, we purchased all of the partnership interests in Midstream Gas Services, L.P., which we refer to as MGS, for approximately $4.7 million in cash and 1,125,416 common units in Eagle Rock Pipeline from a group of private investors, including Natural Gas Partners VII, L.P. We will issue up to 812,540 of our common units, which we refer to as the Deferred Common Units, to Natural Gas Partners VII, L.P., the primary equity owner of MGS, as a contingent earn-out payment if MGS achieves certain financial objectives for the year ending December 31, 2007. Prior to the acquisition, Natural Gas Partners VII, L.P. owned a 95% limited partnership interest in MGS and a 95% interest in its general partner, which owned a 1% general partner interest in MGS. We refer to the private investors who received common units in Eagle Rock Pipeline as partial consideration for the MGS acquisition as the June 2006 Private Investors. The March 2006 Private Investors and the June 2006 Private Investors are collectively referred to in this prospectus as the “Private Investors.” Each of the Private Investors’ common units in Eagle Rock Pipeline will be converted into common units in us upon consummation of this offering on approximately a 1-for-0.732 common unit basis. Because of the contingent nature of the earn-out provision, the information in this prospectus assumes that the Deferred Common Units are not issued.

      Prior to the consummation of this offering, we anticipate entering into an amended and restated credit facility that we expect will provide for an aggregate of $500 million in borrowing capacity. At the closing of this offering:

      The diagram on the following page depicts our organization and ownership after giving effect to the offering and the related formation transactions.

Ownership of Eagle Rock Energy Partners, L.P.

Management of Eagle Rock Energy Partners

      Eagle Rock Energy GP, L.P., our general partner, has sole responsibility for conducting our business and for managing our operations. Because our general partner is a limited partnership, its general partner, Eagle Rock Energy G&P, LLC, will conduct our business and operations, and the board of directors and executive officers of Eagle Rock Energy G&P, LLC will make decisions on our behalf. The senior executives who currently manage our business will continue to do so following the completion of this offering. Neither our general partner, nor any of its affiliates, will receive any management fee or other compensation in connection with the management of our business, but they will be entitled to reimbursement for all direct and indirect expenses they incur on our behalf.

      Neither our general partner nor the board of directors of Eagle Rock Energy G&P, LLC will be elected by our unitholders. Unlike shareholders in a publicly traded corporation, our unitholders will not be entitled to elect the directors of Eagle Rock Energy G&P, LLC. Because of its ownership of a majority interest in Eagle Rock Holdings, L.P., Natural Gas Partners will have the right to elect all of the members of the board of directors of Eagle Rock Energy G&P, LLC at the closing of this offering. References herein to the officers or directors of our general partner refer to the officers and directors of Eagle Rock Energy G&P, LLC. In addition, certain references to our general partner refer to Eagle Rock Energy GP, L.P. and Eagle Rock Energy G&P, LLC, collectively.

      As is common with publicly traded limited partnerships and in order to maximize operational flexibility, we will conduct our operations through subsidiaries. We will initially have one direct subsidiary, Eagle Rock Pipeline, L.P., a limited partnership that will conduct business through itself and its subsidiaries.

      Natural Gas Partners, which will control our general partner, is headquartered in Irving, Texas. Founded in 1988, Natural Gas Partners is among the oldest of the private equity firms that specialize in the energy industry. Through its family of eight institutionally-backed investment funds, Natural Gas Partners has sponsored over 100 portfolio companies and has controlled invested capital and additional commitments totaling $2.9 billion.

Principal Executive Offices and Internet Address

      Our principal executive offices are located at 14950 Heathrow Forest Parkway, Suite 111, Houston, Texas 77032 and our telephone number is (832) 327-8000. Our website is located at www.eaglerockenergy.com. We expect to make our periodic reports and other information filed with or furnished to the Securities and Exchange Commission, which we refer to as the SEC, available, free of charge, through our website, as soon as reasonably practicable after those reports and other information are electronically filed with or furnished to the SEC. Information on our website or any other website is not incorporated by reference into this prospectus and does not constitute a part of this prospectus.

Our General Partner’s Rights to Receive Distributions

      2% General Partner Interest. Our general partner initially will be entitled to receive 2% of our quarterly cash distributions. The general partner’s initial 2% interest in these distributions will be reduced if we issue additional units in the future and our general partner does not elect to contribute a proportionate amount of capital to us to maintain its initial 2% general partner interest. All references in this prospectus to the general partner’s 2% general partner interest assumes that the general partner will elect to make these additional capital contributions in order to maintain its right to receive 2% of these cash distributions.

      Incentive Distributions. In addition to its 2% general partner interest, our general partner holds the incentive distribution rights, which are non-voting limited partner interests that represent the right to receive an increasing percentage of quarterly distributions of available cash as higher target distribution levels of cash have been distributed to the unitholders. The following table shows how our available cash

from operating surplus is allocated among our unitholders and the general partner as higher target distribution levels are met:

      For a more detailed description of the incentive distribution rights, please read “Provisions of Our Partnership Agreement Relating to Cash Distributions — General Partner Interest and Incentive Distribution Rights.”

Summary of Conflicts of Interest and Fiduciary Duties

      General. Eagle Rock Energy GP, L.P., our general partner, has a legal duty to manage us in a manner beneficial to holders of our common units and subordinated units. This legal duty originates in statutes and judicial decisions and is commonly referred to as a “fiduciary duty.” The officers and directors of Eagle Rock Energy G&P, LLC also have fiduciary duties to manage Eagle Rock Energy G&P, LLC and our general partner in a manner beneficial to their owners. As a result of this relationship, conflicts of interest may arise in the future between us and holders of our common units and subordinated units, on the one hand, and our general partner and its affiliates on the other hand. For example, our general partner will be entitled to make determinations that affect our ability to make cash distributions, including determinations related to:

      These determinations will have an effect on the amount of cash distributions we make to the holders of common units, which in turn has an effect on whether our general partner receives incentive cash distributions as discussed above.

      Partnership Agreement Modifications to Fiduciary Duties. Our partnership agreement limits the liability and reduces the fiduciary duties of our general partner to holders of our common units and subordinated units. Our partnership agreement also restricts the remedies available to holders of our common units and subordinated units for actions that might otherwise constitute a breach of our general partner’s fiduciary duties owed to holders of our common units and subordinated units. By purchasing a common unit, the purchaser agrees to be bound by the terms of our partnership agreement and, pursuant to the terms of our partnership agreement, each holder of common units consents to various actions contemplated in the partnership agreement and conflicts of interest that might otherwise be considered a breach of fiduciary or other duties under applicable state law.

      Our general partner’s affiliates may engage in competition with us. Our partnership agreement provides that our general partner will be restricted from engaging in any business activities other than those incidental to its ownership of interests in us. Except as provided in our partnership agreement, Eagle Rock Holdings, L.P. and the NGP Investors are not prohibited from engaging in, and are not required to offer us the opportunity to engage in, other businesses or activities, including those that might be in direct competition with us.

      For a more detailed description of the conflicts of interest and fiduciary duties of our general partner, please read “Conflicts of Interest and Fiduciary Duties.”

The Offering

Summary Historical and Pro Forma Financial Data

      The following table shows summary historical financial data of our predecessor, ONEOK Texas Field Services L.P., and Eagle Rock Pipeline, L.P. and unaudited pro forma financial data of Eagle Rock Energy Partners, L.P. for the periods and as of the dates indicated. ONEOK Texas Field Services, L.P. is treated as our and Eagle Rock Pipeline, L.P.’s predecessor and is referred to as “Eagle Rock Predecessor” throughout this prospectus because of the substantial size of the operations of ONEOK Texas Field Services, L.P. as compared to Eagle Rock Pipeline, L.P. and the fact that all of Eagle Rock Pipeline, L.P.’s operations at the time of the acquisition of ONEOK Texas Field Services, L.P. related to an investment that was managed and operated by others. References in this prospectus to “Eagle Rock Pipeline” refer to Eagle Rock Pipeline, L.P., which is the acquirer of Eagle Rock Predecessor and the entity contributed to Eagle Rock Energy Partners, L.P. in connection with this offering.

      Our historical results of operations for the periods presented below may not be comparable either from period to period or going forward, for the following reasons:

      The summary historical financial data for the year ended December 31, 2003, as of and for the year ended December 31, 2004 and as of and for the eleven month period ended November 30, 2005 are derived from the audited financial statements of Eagle Rock Predecessor and as of and for the years ended December 31, 2003, 2004 and 2005 are derived from the audited financial statements of Eagle Rock Pipeline. The summary historical financial data as of December 31, 2003 is derived from the unaudited financial statements of Eagle Rock Predecessor. The summary historical financial data for the six months ended June 30, 2005 and as of and for the six months ended June 30, 2006 are derived from the unaudited financial statements of Eagle Rock Pipeline. The summary pro forma financial data for the year ended December 31, 2005 and as of and for the six months ended June 30, 2006 are derived from the unaudited pro forma financial statements of Eagle Rock Energy Partners, L.P. The pro forma adjustments have been prepared as if this offering and certain transactions to be effected at the closing of this offering had taken place as of June 30, 2006 in the case of the pro forma balance sheet or as of January 1, 2005, in the case of the pro forma statements of operations for the year ended December 31, 2005 and the six months ended June 30, 2006. For a description of the pro forma adjustments included in the following table, please read the pro forma financial statements included in this prospectus.

      The following table includes the non-GAAP financial measures of EBITDA, Adjusted EBITDA and segment gross margin. We define EBITDA as net income plus interest expense, net, provision for income taxes and depreciation and amortization expense. We define Adjusted EBITDA as net income plus interest expense, net, provision for income taxes and depreciation and amortization expense, less the impact of unrealized derivatives gains (losses), less income from discontinued operations. We believe Adjusted EBITDA more accurately reflects our current operations’ ability to generate cash flows independent of capital structure and of the fluctuations in unrealized, mark-to-market adjustments which are by their nature volatile and not reflective of the underlying operations. In addition, as unrealized gains/losses, they are not components of distributable cash. We define segment gross margin as total revenue less cost of gas and liquids and other cost of sales. For a reconciliation of EBITDA, Adjusted EBITDA and segment gross margin to their most directly comparable financial measures calculated and presented in accordance with GAAP (accounting principles generally accepted in the United States), please read “— Non-GAAP Financial Measures.”

Non-GAAP Financial Measures

      We include in this prospectus the following non-GAAP financial measures: EBITDA, Adjusted EBITDA and segment gross margin. We provide reconciliations of these non-GAAP financial measures to their most directly comparable financial measures as calculated and presented in accordance with GAAP.

      We define EBITDA as net income plus interest expense, net, provision for income taxes and depreciation and amortization expense. EBITDA is used as a supplemental liquidity measure by our management team and by external users of our financial statements such as investors, commercial banks, research analysts and others to assess the ability of our assets to generate cash sufficient to pay interest costs, support our indebtedness, make cash distributions to our unitholders and general partner and finance maintenance capital expenditures. EBITDA is also used as a supplemental measure by management and by external users of our financial statements such as investors, commercial banks, research analysts and others, to assess:

      We define Adjusted EBITDA as net income plus interest expense, net, provision for income taxes and depreciation and amortization expense, less the non-cash, mark-to-market impact of unrealized derivatives gains (losses), less income from discontinued operations deemed as non-recurring impacts. Adjusted EBITDA is useful in determining our ability to sustain or increase distributions. By excluding unrealized derivative gains (losses), a non-cash charge that represents the change in fair market value of our executed derivative instruments and is independent of our assets’ performance or cash flow generating ability, Adjusted EBITDA reflects more accurately our ability to generate cash sufficient to pay interest costs, support our level of indebtedness, make cash distributions to our unitholders and general partner and finance our maintenance capital expenditures. Adjusted EBITDA also describes more accurately the underlying performance of our operating assets by isolating the performance of our operating assets from the impact of an unrealized, non-cash measure designed to describe the fluctuating inherent value of a financial asset. Similarly, by excluding the impact of non-recurring discontinued operations, Adjusted EBITDA provides users of our financial statements a more accurate picture of our current assets’ cash generation ability, independently from that of assets that are no longer a part of our operations.

      Neither EBITDA nor Adjusted EBITDA should be considered an alternative to net income, operating income, cash flows from operating activities or any other measure of financial performance presented in accordance with GAAP.

      Neither EBITDA nor Adjusted EBITDA includes interest expense, income taxes or depreciation and amortization expense. Because we have borrowed money to finance our operations, interest expense is a necessary element of our costs and our ability to generate segment gross margins. Because we use capital assets, depreciation and amortization are also necessary elements of our costs. Therefore, any measures that exclude these elements have material limitations. To compensate for these limitations, we believe that it is important to consider both net earnings determined under GAAP, as well as EBITDA, to evaluate our liquidity. Our EBITDA and Adjusted EBITDA excludes some, but not all, items that affect net income and operating income and these measures may vary among companies. Therefore, our EBITDA and Adjusted EBITDA may not be comparable to similarly titled measures of other companies.

      We define segment gross margin as total revenues less cost of natural gas and NGLs and other cost of sales. Segment gross margin is included as a supplemental disclosure because it is a primary performance measure used by management as it represents the results of product sales and purchases, a key component of our operations. As an indicator of our operating performance, segment gross margin should not be considered an alternative to, or more meaningful than, net income as determined in accordance with GAAP. Our segment gross margin may not be comparable to a similarly titled measure of another company because other entities may not calculate segment gross margin in the same manner.