Underfloor air distribution (UFAD) systems are innovative methods for delivering space conditioning in offices and other commercial buildings. Underfloor air distribution derives its name from the use of the underfloor plenum below a raised (access) floor system to supply conditioned air directly into the occupied zone of the building, typically through floor diffusers. The use of UFAD technology is increasing in North America because of the benefits that it offers over conventional overhead air distribution. The purpose of this design guide is to provide assistance in the design of UFAD systems that are energy efficient, intelligently operated, and effective in their performance. This guide also describes important research results that support current thinking on UFAD design and includes an extensive annotated bibliography for those seeking additional detailed information. This guide does not cover conventional overhead air distribution system design procedures in depth but rather focuses on the major differences between UFAD systems and conventional design. For more information on standard heating, ventilating, and air-conditioning (HVAC) design, please refer to other books published by ASHRAE, including the Handbook series [ASHRAE 2000, 2001a, 2002, 2003a], Air-Conditioning Systems Design Manual [ASHRAE 1993], and Designer’s Guide to Ceiling- Based Air Diffusion [Rock and Zhu 2001].