The circadian clock is a self-sustaining oscillator that has a period of approximately 24 h and controls many physiological and behavioral systems. This clock can synchronize itself to changing environmental conditions to optimize an organisms performance. The underlying circadian rhythms are generated by periodic activation of transcription by a set of clock genes. Besides their own regulation, clock genes can influence biochemical processes by modulating specific genes of biochemical pathways. Developments in the last few years using genetics and molecular biological tools have led to a new understanding of the molecular basis of the circadian clock in mammals. In this mini-review, I will summarize these advances that have led us to begin understanding the mammalian circadian clock at the molecular level.