The changing U.S. energy market continues to have a dramatic effect on the construction and operation of oil and gas pipeline infrastructure. One of the largest new pipeline construction projects in the past several decades – the nearly 1,700 mile Rockies Express gas pipeline between Wyoming and Ohio (REX) – began operation in 2009, with a west to east direction of flow. The rapid development of Marcellus and Utica shale plays changed the market demands quickly, however, and on November 26, 2013, FERC approved REX’s request to reverse flow (east to west) for part of its system. A large number of existing pipelines (both gas and oil) have been undertaking reversal of flow and conversion of service for market reasons, following the rapid production of tight oil and gas around the U.S. At present, there are no express federal regulations that address reversal of flow, beyond FERC rate issues. Reversals and conversions may trigger other PHMSA regulatory obligations, however, such as IMP risk and impact assessments. Environmental considerations (such as permitting and NEPA) may also be triggered. Pipelines that are converted to transport a new product such as natural gas to liquid or vice versa must comply with PHMSA regulatory requirements at 49 CFR Parts 195.5 or 192.14, including review of construction and operational records and performing appropriate tests where records are unavailable, in addition to environmental considerations.