Maize environments in developing countries can be grouped into four major classifications: tropical, subtropical, temperate, and highland (Table 3.1). These classifications do not correspond precisely to geographic definitions of the tropics, subtropics, and temperate regions (Fig. 3.1). Rather they are based on agroclimatic criteria: (1) minimum and maximum mean temperatures during the growing season, (2) altitude above sea level, and (3) to a lesser extent, latitude (Table 3.2).