The purpose of this collection of 27 papers is to introduce the subject of underbalanced drilling to engineers interested in gaining a basic overview of the technology and its applications. It is intended not to offer a comprehensive look at the topic but, rather, to provide the reader with enough knowledge to allow for more advanced study or work in the field. Topics covered include: • UBD Background and Overview • Candidate Selection and Reservoir Issues • Flow Modeling • Operational Aspects • Safety • Coiled Tubing • Well Control • Well Completions.