Introduction Data centers and telecommunications rooms that house datacom equipment are becoming increasingly more difficult to adequately cool. This is a result of IT manufacturers increasing datacom performance year after year at the cost of increased heat dissipation. Even though performance has@ in general@ increased at a more rapid rate than power@ the power required and the resulting heat dissipated by the datacom equipment has increased to a level that is putting a strain on data centers. However@ in the struggle to improve the thermal management characteristics of data centers it is sometimes important to assess today's data center designs. The objective of this book is to provide a series of case studies of high density data centers and a range of ventilation schemes that demonstrate how loads can be cooled using a number of different approaches. This introductory chapter describes the various ventilation designs most often employed within data centers. This book does not present an exhaustive resource for existing ventilation schemes but@ rather@ a wide variety of schemes commonly used in the industry. Seven primary ventilation schemes are outlined here. In the case studies that follow@ each of these will be shown with detailed measurements of airflow@ power@ and temperature. The most common ventilation design for data centers is the raised-access floor supply@ with racks arranged in a cold-aisle/hot-aisle layout (see Figure 1.1). The chilled-air supply enters the room through perforated tiles in the raised floor@ washing the fronts of the racks facing the cold aisle. The hot exhaust air from the racks then migrates back to the inlet of the computer room air-conditioning units (CRACs) typically located on the perimeter of the data center. Another version of the raised-access floor supply is shown in Figure 1.2@ where the air-handling units (AHUs) are located beneath the floor containing the IT equipment. One of the key advantages of this arrangement is that all the mechanical equipment is located in a room separate from the IT equipment@ which allows for ease of maintenance.