Purpose The purpose of these recommended practices is to provide information that can serve as a guide for planning@ installation@ operation and testing of underbalanced drilling equipment systems on land and offshore drilling rigs [barge@ platform@ bottom-supported@ and floating with surface blowout preventers (BOP) installed] thereby ensuring consideration of personnel safety@ public safety@ integrity of the underbalanced drilling (UBD) equipment@ and preservation of the environment for onshore and offshore UBD operations (including tripping of drill string). The UBD system is composed of all equipment required to safely allow drilling ahead in geological formations with pressure at surface and under varying rig and well conditions. These systems include: the rig circulating equipment@ the drill string@ drill string non return valves (NRV)@ surface BOP@ control devices (rotating or non-rotating) independent of the BOP@ choke and kill lines@ UBD flowlines@ choke manifolds@ hydraulic control systems@ UBD separators@ flare lines@ flare stacks and flare pits and other auxiliary equipment. The primary functions of these systems are to contain well fluids and pressures within a design envelope in a closed flow control system@ provide means to add fluid to the wellbore@ and allow controlled volumes to be withdrawn from the wellbore. Managed pressure drilling (Category A) and mud cap drilling (Category C) techniques as defined in the IADC Well Classification System for Underbalanced Operations and Managed Pressure Drilling are not included in this publication. The phrase managed pressure drilling or the acronym MPD is only used in this document in the context of the IADC Well Classification System. Sub-sea BOP stacks and marine risers are not dealt with in this document.