The review addresses different types of dormancy on fish. It can be subdivided into two groups. The first is adult aestivation. This allows fish to survive droughts in low latitude freshwater environments (several species from various families), or ice coverage periods causing anoxic conditions in high latitudes (cyprinid Carassius spp.). It is a facultative phenomenon induced by various environmental cues (but not photoperiods), accompanied by specific biochemical adaptations that slow down the metabolism of aestivating individuals and allow their bodies to tolerate pollution from metabolic waste products. The second, more variable group is embryonic diapauses. These obligatory processes are well-integrated into the life cycle of fishes. It occurs in more than 30 killifish species (order Cyprinodontiformes) as a key adaptation that significantly expands their environmental range to include ephemeral pools. Diapause in several elasmobranch species occurs in the mother's body. In bitterling Acheilognathus, diapause is a key adaptation to effectively use their limiting resource-the large bivalves where bitterlings deposit their eggs. Cessation of female gonad development after completing vitellogenesis is common in many species of high latitudes. This phenomenon has not been considered in the context of fish dormancy before, but biologically it fulfills the same functions as embryonic diapause and thus should be considered within the same framework. The effectiveness of life strategies containing diapause depends very much on both the sensitivity of dormant organisms to environmental stresses and the accuracy of their responses to environmental cues-primary changes in photoperiod or temperature to induce or terminate dormancy. The optimal strategy should balance phenotypic plasticity and bet-hedging as tools to maximize the fitness of diapausing organisms in partially predicted environments.