The following is a brief description of the major domestic and international breeding and testing nurseries of the CIMMYT Bread Wheat Program written for the purpose of acquainting visiting scientists and trainees with the role of each nursery in CIMMYT's breeding scheme, how it is planted and what relationships it might have to other CIMMYT nurseries, both domestic and international. This explanation is not an attempt to explain CIMMYT breeding philosophies and is not a complete listing of all nurseries or all locations with which CIMMYT works. CIMMYT breeding nurseries generally have Spanish names. Consequently, their abbreviations do not exactly match their English translations. International nurseries are given English names. The CIMMYT Bread Wheat Program operates primarily in three Mexican locations. Ciudad Obregon (elevation 40 meters, 29° latitude north, temperature range from 0°C to 40°C) is located in the State of Sonora and is utilized for he winter breeding cycle. Cd. Obregon offers environmental conditions which are conductive to maximum expression of yield potential. The screening of breeding material for leaf rust resistance is facilitated by the annual leaf rust epidemic which occurs throughout the Yaqui Valley. Cd. Obregon is a desert location and therefore is irrigated throughout the cropping season.