Most organisms adapt the timing of their physiology to the cyclic changes of the environment with the use of intrinsic time-keeping systems called circadian clocks. Central features of the molecular clock mechanism are transcription- and translation-based negative feedback loops: clock genes and their products interact to generate oscillation of specific transcripts and proteins, and, ultimately, circadian rhythmicity and behavior. Various transcription factors constitute the molecular clock, and the signal transduction cascades governing their function appear to be crucial for the fine-tuning of the circadian cycle.