Our highly seasonal world imposes environmental challenges for insects. To survive these inimical periods they rely on a diapause (dormancy) mechanism to bridge unfavorable seasons. The origin of the term “diapause” is discussed, as well as its relationship to related forms of dormancy in other animals. Diapause is distinct from quiescence in that it is not an immediate response to an adverse environment but is programmed at an earlier developmental stage, an attribute that enables the insect to take steps in preparation for entering the arrested state. Diapause can occur at any point in the life cycle (embryo, larva, pupa, adult), but when it occurs it is species-specific. The chapter summarizes who does it and in what stage, as well as addressing the occurrence of diapause in social insects. The pervasive impact of diapause on the insect life cycle begins prior to diapause and continues well beyond its termination.