Potassium (K) deficiency in cotton has long been a severe limitation in large areas of California, and recent reports indicate that K deficiency is becoming more prevalent on cotton in the southern states. The sensitivity of cotton to late season K deficiency on soils that are not K deficient for other crops reflects the dominance of developing fruit as a sink for K, and incongruent patterns of root distribution and available K in the soil profile. Unlike most other annual crop species, the cotton root system is characterized by poor root development in the surface soil layer where available soil K is concentrated. This rooting habit suggests that the cotton plant is more dependent on subsurface soil K supply, but subsurface soils are commonly K deficient in many cotton production areas. There are, however, large differences in K use efficiency between cotton cultivars that are associated with considerable differences in root growth and the rate of K uptake from soil. Components of an integrated management strategy for dealing with cotton K deficiency are discussed. Both lint yield and fiber quality are dependent on adequate K nutrition of the cotton crop.