Book excerpt: Now that many of the clock genes have been identified it is possible to track daily patterns of clock-related mRNAs and proteins to link the entraining light cycles with molecular oscillations within the cell. Insect experiments have led the way in demonstrating that the concept of a "master clock" can no longer be used to explain the temporal organization within an animal. Insects have a multitude of cellular clocks that can function independently and retain their function under organ culture conditions, and they thus offer a premier system for studying how the hierarchical organization of clocks results in the overall temporal organization of the animal. Photoperiodism, and its most obvious manifestation, diapause, does not yet have the molecular underpinning that has been established for circadian rhythms, but recent studies are beginning to identify genes that appear to be involved in the regulation of diapause.