Publisher Summary This chapter discusses circadian rhythms in insects. Circadian rhythms are daily oscillations in physiology, metabolism, or behavior that persist (freerun) in organisms that have been isolated from periodic fluctuations in the environment. There are four transcriptional regulators genes which are crtical for generating the basic circadian oscillation, the period (per), timeless (tim), cycle (cyc), and clock (clk). There are 4 essential elements–—a pacemaker or oscillator that generates the primary timing signal, photoreceptors for entrainment, and two coupling pathways, one that mediates the flow of entrainment information from the photoreceptor to the pacemaker, and another that couples the pacemaker to the effector mechanisms that it controls. In insects, the circadian system is responsible for imposing daily rhythmicity on a wide variety of processes including locomotor activity, mating, oviposition, egg hatching, pupation and pupal eclosion, pheromone release, retinal sensitivity to light, olfactory sensitivity, and even learning and memory. Studies on the anatomical and physiological organization of circadian systems in insects have largely focused on behavioral rhythms and their control by the nervous system. The goal of these studies has been to identify tissues and cells that comprise the functional defined components of the circadian system.