SYNOPSIS. The initiation of a molting cycle in insects is neither a random nor a strictly periodic event. Insofar as molting can accomplish different things under different circumstances, such as a change in size or a change in form, it is reasonable to asume that the timing of a molt must be adapted to these functions. The onset of a metamorphic molt, in particular, must be precisely controlled because the onset of metamorphosis terminates the growth phase of a larva and establishes the body size of the adult insect. This aspect of the control of molting has received relatively little attention and our knowledge of specific physiological mechanisms for the control of molt initiation is restricted to three species: the blood-sucking bug, Rhodnius prolixus, the greater milkweed bug, Oncopeltus fasciatus, and the tobacco hornworm, Manduca sexta. The present review discusses the state of our knowledge about the factors that render these insects competent to molt and about the stimuli that serve as a direct trigger for molting.