Atlas Energy, L.P. engages in the development and production of natural gas and oil in the Appalachian basin, Illinois basin, and the Rocky Mountain region of the United States. It is also involved in the provision of natural gas gathering, processing, and treating services in the Anadarko and Permian basins located in the southwestern and mid-continent regions of the United States; natural gas gathering services in the Appalachian basin in the northeastern region of the United States; and natural gas liquids transportation services in the southwest region of the United States. As of December 31, 2011, the company had working interests in approximately 8,500 gross producing natural gas and oil wells; overriding royalty interests in approximately 500 gross producing natural gas and oil wells; and proved reserves of 167.6 billion cubic feet equivalent. It operated 7 active natural gas processing plants with aggregate capacity of approximately 610 MMcfd; 9,000 miles of active natural gas gathering systems located in Oklahoma, Kansas, and Texas, which transport gas from wells and central delivery points in the Anadarko and Permian basins; and 100 miles of active natural gas gathering systems located in Tennessee. Atlas Energy GP, LLC serves as a general partner of Atlas Energy, L.P. The company was formerly known as Atlas Pipeline Holdings, L.P. and changed its name to Atlas Energy, L.P. in February 2011. Atlas Energy, L.P. is based in Pittsburgh, Pennsylvania.