This guideline for the preparation of operating and main- tenance (O&M) documentation for building systems complements ASHRAE Guideline 1-1989, HVAC Commissioning Process. The guideline is directly applicable to HVAC&R systems but can also be used for other building systems. Accurate and relevant O&M documentation is essential for the safe, reliable, and efficient operation of buildings and building systems. Timely delivery of O&M documentation is also important because the documentation is initially needed during commissioning. This guideline addresses the requirements of accuracy, relevancy, and timeliness. It presents a comprehensive and systematic approach to the,preparation of O&M documentation to effectively support the development and management of O&M programs in buildings.