The expression of rhythmicity in insects has been largely documented at the cellular level, in organs, as changes in the physiology and behaviour of individuals, as well as in population synchrony. Circadian rhythms are particularly ubiquitous in insects and, as in other organisms, have been the best analysed. Fruit flies, cockroaches, butterflies, honey bees, and other insects cast much light on the physiology and molecular basis of the circadian clock. Insect adaptation and success in the colonisation of the most diverse environments is also associated with the temporal organisation of their daily life. In this chapter, we will analyse how circadian rhythms are expressed and controlled in insects. Classical and recent findings will be discussed from an adaptive and integrative perspective.