The ISPE Baseline Guide® Water and Steam Systems (Third Edition) aims to assist with the design, construction, operation, and lifecycle management of new and existing water and steam systems. It is intended to help meet Good Manufacturing Practices (GMPs) and comply with regulations and related guidance. This latest version describes new variations in the European Pharmacopoeia for the manufacture of Water for Injection by methods other than distillation. Additional changes include discussions on the global harmonization of water quality attributes, comprehensive pretreatment design, rapid microbial monitoring, ozone for ambient sanitization, and membrane technologies. This version is aligned with the ISPE Baseline® Guide: Commissioning and Qualification (Second Edition). The Guide was written by a global team of critical utilities experts with a combined experience of more than 500 years. Much of the team responsible for the earlier versions of the Water and Steam Systems Baseline Guide has returned to contribute to the revised Guide, providing continuity and longevity of vision to the Guide's contents.