Irrigation is a major management consideration in cotton production. The plants require water delivered at intervals through 65 to 85 percent of the growing season. Not only is water a significant production cost, but its regulation through proper scheduling provides a unique opportunity to control plant growth and development in a way that favors high productivity. Such regulation requires an understanding of how cotton responds to water. This report summarizes several long-term cotton irrigation studies in the San Joaquin Valley. The results apply to conventional plantings with rows spaced 38 to 40 inches apart and normal plant populations.