Understanding the effect of heat treatment on the properties and function of casein is important for its use in food processing. However, fewer studies on the effects of heat treatment on casein under acidic conditions have been reported compared with those under neutral conditions. In this review, heat-induced aggregation and gel formation under acidic conditions were described, and then the thermal behavior characteristics of casein under acidic conditions were discussed. The behaviors of casein toward heat and salts under acidic conditions were markedly different from those under neutral conditions. It was impossible to explain those behaviors from a viewpoint of properties, conformation, and interactions of casein. Further studies on the thermal behavior characteristics of casein under acidic conditions are needed for clarification of the mechanisms involved.