Many insect species live in the arctic and show a wide range of adaptations to its extreme severity and seasonality. Long, cold winters are met, for example, by cold hardiness and choice of protected sites. Cold hardiness includes both widespread tolerance to freezing and extreme supercooling ability, as well as unusual responses in a few species, such as lack of typical cryoprotectants. Adaptations to short, cool summers include activity at low temperatures, selection of warm habitats and microhabitats, melanism and hairiness coupled with basking behaviour, and prolonged or abbreviated life cycles. Diapause ensures that many species emerge early in summer, with brief synchronized reproduction that maximizes the time for offspring development before winter returns. Some species overwinter in sites that thaw earliest in spring, even if they are relatively exposed in winter. Other adaptations respond to year-to-year variability: for example, prolonged diapause can provide insurance against unsuitable summers. All of these adaptations are co-ordinated. For example, cold hardiness relies on physiological and biochemical adaptations but also on habitat choice and timing. Because the adaptations are complex, predicted climatic warming probably will have unexpected effects. In particular, an increase in temperature that increases summer cloud when sea ice melts would likely reduce temperatures for insect development and activity, because sunshine provides critical warmth to insects and their microhabitats. Changes in moisture will also be important. Moreover, responses differ among species, depending especially on their microhabitats. The complexity of the responses of insects to arctic conditions reinforces the need for research that is sufficiently detailed.