The daily rotation of the earth on its axis leads to predictable periodic fluctuations of environmental conditions. Accordingly, most organisms have evolved an internal timing mechanism, the circadian clock, which is able to recognize these 24-hour rhythmic oscillations. In plants, the temporal synchronization of physiology with the environment is essential for successful plant growth and development. The intimate connection between light signaling pathways and the circadian oscillator allows the anticipation of the environmental transitions and the measurement of day-length as an indicator of changing seasons. In recent years, significant advances have been made in the genetic and molecular dissection of the plant circadian system, mostly in Arabidopsis thaliana. The overall plant clock organization is highly complex; the system seems to include several input pathways, tightly regulated central oscillators and a myriad of outputs. The molecular cloning and characterization of a number of clock components has greatly improved our view of the plant central oscillator and additional players will most likely come into place very soon. Molecular mechanisms underlying circadian clock function are also beginning to be characterized. The emerging model relies on negative feedback loops at the core of the oscillator. Additional levels of post-transcriptional and post-translational regulation also contribute to the generation and maintenance of the rhythms. Globally, these studies have shed new light on how the clock coordinates plant physiology and development with the daily and seasonal environmental cycles.