Diapause can be thought of as a kind of time travel in which animals cease activity at one point only to resume it later, often after a long period of inactivity. The capacity to enter diapause is common to many, although not all, organisms. Investigation of the phylogenetic distribution of diapause among all organisms, all animals, or even a well-defined clade such as the Crustacea, would do much to help in determining whether diapause has evolved only once and has been lost many times, or because it has evolved independently many times. For this we must have access to both stable phylogenies and a reliable means of determining the homology of the various expressions of diapause. Both are serious challenges to the field. Because of the high short-term fitness cost of diapause, the trait is almost certainly adaptive in most instances in which it is present. Tn any given instance, diapause might serve as a mechanism for avoiding either predictable or unpredictable harsh periods. An alternative suggestion that diapause is an adaptation to specialize on the most favorable periods in the environment is inconsistent with current theory. The nature of adaptation, theoretical predictions about optimal trait expression, and tests of theory will all be constrained by the physiological capabilities of the organisms. Understanding these constraints is essential for understanding adaptation. Tn theory, possession of diapause can affect both evolutionary and ecological dynamics in important ways by fostering the coexistence of genotypes within populations and species within communities. Ecosystem-level processes may be affected if animals entering and emerging from diapause early with them important trophic-dynamic or nutrient-cycling functions. Central to understanding these roles is the determination of the amount of generation overlap produced by prolonged diapause. However, quantifying average generation overlap is challenging because different individuals experience widely varying exposures to the cues to emerge from diapause, experience differing sources and levels of mortality while in diapause, and may have distinct pre-programmed diapause durations. Diapausing stages, when present in aquatic sediments that are structured in historical order, have the potential to provide insights into the evolutionary and ecological past of a population. Efforts to extract information on molecular genetics and organism function are only just beginning. The study of diapause in a well-defined taxon such as the Crustacea provides opportunities for linking understanding of such distinct fields of study as phylogenetics, population genetics, ecological genetics, community ecology, and ecosystem dynamics.