Renowned drilling expert Bill Rehm of Far East Energy provides a detailed view of underbalanced drilling operations with terms, calculations, and guidelines. Discusses surface control equipment, downhole tools, gases and equipment, circulation and the fluid column, flow and mud-cap drilling, liquid-gas fluids, foam drilling, and air-gas, mist, and foamed-mist drilling. Also discusses the problems of corrosion and scale and addresses rigging up, flares, and flaring. The current IADC WellCAP curricu-lum for underbalanced drilling is included as an appendix, along with a glossary and bibliography.