JH has multiple functions and a primary role of JH in insect development is to modulate ecdysone action. It is principally a morphogenetic hormone. Its action consists in reconstitution of the capability for growth of parts of the body which have lost it in the course of morphogenesis. All other changes observed in the body after the administration of JH are indirect consequences of this action and are due to the altered biochemical balance produced in the body by the changed proportion of metabolically active tissue. JH maintains the current commitment of the tissues and cells, whereas ecdysone causes both predifferentiative and differentiative cellular events that are necessary for the moult. Thus, when JH is present, a moult to a larval stage ensues. If JH is absent at the onset of the moult, metamorphosis occurs. Studies of the molecular mechanisms of JH action have been hampered by the failure to isolate its receptor(s). Recently, detailed studies of JH action have begun, and further studies should elucidate new aspects of the action of this unique molecule.