A combination of forward and reverse genetic approaches together with transcriptome-scale gene expression analyses have allowed the elaboration of a model for the Arabidopsis thaliana circadian clock. The working model largely conforms to the expected negative feedback loop model that has emerged from studies in other model systems. Although a core loop has emerged, it is clear that additional components remain to be identified and that the workings of the Arabidopsis clock have been established only in outline. Similarly, the details of resetting by light and temperature are only incompletely known. In contrast, the mechanism of photoperiodic induction of flowering is known in considerable detail and is consistent with the external coincidence model.