The need for more performance from computer equipment in data centers has driven the power consumed to levels that are straining thermal management in the centers. When the computer industry switched from bipolar to CMOS transistors in the early 1990s, low-power CMOS technology was expected to resolve all problems associated with power and heat. However, equipment power consumption with CMOS has been rising at a rapid rate during the past 10 years and has surpassed power consumption from equipment installed with the bipolar technologies 10 to 15 years ago. Data centers are being designed with 1520-year life spans, and customers must know how to plan for the power and cooling within these data centers. This paper provides an overview of some of the ongoing work to operate within the thermal environment of a data center. Some of the factors that affect the environmental conditions of data-communication (datacom) equipment within a data center are described. Since high-density racks clustered within a data center are of most concern, measurements are presented along with the conditions necessary to meet the datacom equipment environmental requirements. A number of numerical modeling experiments have been performed in order to describe the governing thermo-fluid mechanisms, and an attempt is made to quantify these processes through performance metrics.