This reprint, which focuses on the best-practices applications of the technology, includes such topics as introduction to underbalanced operations; case histories; underbalanced operations other than drilling; well design; equipment and health, safety, and environment; and fluids and modeling. Included is an overview of the candidate-selection process and the design criteria used for underbalanced drilling operations. Additional topics include other applications for this technology, successful design implementation, fluid optimization, and multiphase-flow modeling and simulation.