1. INTRODUCTION In insects, hormones are as important in co-ordinating development and physiology as in vertebrates. But in general, the insect endocrine system is simpler, and produces fewer hormones, than that in the vertebrates. It is possible, therefore, that the study of insect endocrinology may the more easily yield information about the fundamental mechanisms of hormone action. In an article of this length, it is impossible to discuss comprehensively all the present information about endocrine mechanisms in insects. Instead, this review will be concerned mainly with two aspects of insect endocrinology which may have greater appeal to the majority of the readers of this Journal: the control of the activity of the various parts of the insect endocrine system, and the ways in which the hormones act, and interact, during growth and development. 2. COMPOSITION OF THE INSECT ENDOCRINE SYSTEM The insect endocrine system has four major components: groups of