The ASHRAE TC9.9 Thermal Guidelines are widely regarded as the industry standard for establishing IT environmental design conditions. IT equipment manufacturers’ thermal engineers are the primary contributors to the technical content of the guidelines, addressing equipment capabilities and challenges, issues facing the industry, and trends forecast by these same manufacturers. As trends evolve and ASHRAE-sponsored research is completed, the Thermal Guidelines are periodically updated. In March of 2021, ASHRAE TC9.9 published the Fifth Edition of the Thermal Guidelines, which incorporates a number of changes based on ASHRAE research and industry trends, covering both air-cooled and liquid-cooled IT applications and deployments. This session will provide an overview of the changes that have been made to ASHRAE’s Fifth Edition of Thermal Guidelines, and discuss the impacts they can have on new and existing mission critical facilities.