The review presented here shows that a number of eyestalk substances having pronounced physiological effects as endocrine agents can be separated from crude extracts of eyestalk material by various treatments. The hormones appear to be proteinaceous in nature, with several of them apparently polypeptides of relatively low molecular weight. Some of the reported pigmentary-effector hormones are recognizable as distinct molecular species on the basis of chromatographic behavior and molecular size. The diabetogenic hormone is a much larger molecule and is a protein. The properties of the molt-inhibiting hormone of the eyestalk, although still too little known for specific classification, indicate that it belongs to one of these two categories. Tests of functional specificity and structural studies, which in most cases depend on further chemical purification, seem attainable in the near future. The subsequent comparative studies that may become feasible will be of physiological and biochemical interest.