Because of their hidden nature, critical utility systems are vulnerable to non-compliance. With their potential to impact product quality or performance in a significant way, critical utilities consistently receive close scrutiny by regulatory inspectors. The goal of the ISPE Good Practice Guide: Critical Utilities GMP Compliance – How to Be Compliant and Ready to Prove It is to help pharmaceutical organizations achieve and maintain their critical utility systems in a state of control, and then be able to efficiently demonstrate their systems’ Good Manufacturing Practice (GMP) compliance to regulatory inspectors and auditors. To help meet this goal, this Good Practice Guide provides best practices with respect to GMP compliance of critical utility systems. This Guide covers water systems, pure steam, compressed air, and medical gases, as well as processes unique to critical utilities, such as maintaining piping layout drawings and avoiding poor sampling techniques at points of use. Specific critical utility inspection and audit preparation activities are presented, as are broadly applicable best practices, resulting in a comprehensive Good Practice Guide useful in pharmaceutical manufacturing facilities. ISPE Good Practice Guide: Critical Utilities GMP Compliance – How to Be Compliant and Ready to Prove It serves as a companion document to other ISPE Guides, for example, ISPE Baseline® Pharmaceutical Engineering Guide, Volume 4 – Water and Steam Systems.