To use information inherent in the seasonal change in daylength, an insect must be able to measure daylength, distinguish long from short days, count the days, store that information in the brain, and then act on that information at the correct developmental stage to engage the diapause program. This chapter explains the nature of the photoperiodic signal, when it is received, where this information is stored, and how that information triggers the hormonal response. Formal models and molecular approaches examining the role of circadian clock genes consistently point to a circadian basis for the photoperiodic clock. Thermoperiod sometimes substitutes for photoperiod, suggesting alternative pathways for evoking the diapause program. Some tropical insects rely exclusively on temperature or rainfall as environmental signal regulating diapause. Input from hosts can also be important for plant-feeders and parasitoids. The diapause decision is sometimes relegated to the mother, thus requiring an intriguing mechanism for the transfer of environmental information across generations.