1. INTRODUCTION Our knowledge of insect hormones is expanding rapidly and, in general, reviews of the literature have been preoccupied with morphogenetic hormones. A review of this length must, of necessity, be selective. We have avoided much speculative material and disputes about detail, and attempted to give a critical account of the available information concerning the nature and role of neurosecretory hormones in insects. Particular attention has been paid to the physiology of hormones, produced by neurosecretory cells and associated glandular tissue, which are concerned with developmental, physiological and metabolic events. 2. SOURCES OF HORMONES In insects, polypeptide or peptide hormones are produced by neurosecretory cells or glandular cells associated with the neuroendocrine system. The neurosecretory cells may lie within the brain, the corpora cardiaca, or the medial nervous