T he purpose of this standard is to provide requirements on the installation and testing of blowout prevention equipment systems on land and marine drilling rigs (barge, platform, bottom-supported, and floating). Blowout preventer equipment systems are comprised of a combination of various components. The following components are required for operation under varying rig and well conditions: a) blowout preventers (BOPs); b) choke and kill lines; c) choke manifolds; d) control systems; e) auxiliary equipment. The primary functions of these systems are to confine well fluids to the wellbore, provide means to add fluid to the wellbore, and allow controlled volumes to be removed from the wellbore Diverters, shut-in devices, and rotating head systems (rotating control devices) are no longer addressed in this standard (see API 64 and API 16RCD, respectively) as their primary purpose is to safely divert or direct flow rather than to confine fluids to the wellbore. The 4th edition of this document is being published as a Standard as opposed to a Recommended Practice to bring additional consistency to the industry on how BOP systems are inspected, maintained, tested and operated. In this revision, consideration was given to recommendations made by the Joint Industry Equipment Task Force and other organizations following the Macondo incident. International oil and gas operators, manufacturers, and contractors participated in this revision and incorporated those recommendations that were pertinent to the standard. API 53 focuses on those items specific to BOP equipment systems for drilling operations and is reorganized into major sections that contain provisions for surface BOP systems, subsea BOP systems, and those that apply to both. The API Standards Program, which dates back to the 1920's, is accredited by the American National Standards Institute (ANSI) and provides an ongoing forum for continuous improvement by, and collaboration among, companies, regulators, and other industry stakeholders. Over 100 API E&P standards are referenced in government regulations and raise the level of safety performance across the industry.