The circadian clock regulates diverse aspects of plant growth and development and promotes plant fitness. Molecular identification of clock components, primarily in Arabidopsis, has led to recent rapid progress in our understanding of the clock mechanism in higher plants. Using mathematical modeling and experimental approaches, workers in the field have developed a model of the clock that incorporates both transcriptional and posttranscriptional regulation of clock genes. This cell-autonomous clock, or oscillator, generates rhythmic outputs that can be monitored at the cellular and whole-organism level. The clock not only confers daily rhythms in growth and metabolism, but also interacts with signaling pathways involved in plant responses to the environment. Future work will lead to a better understanding of how the clock and other signaling networks are integrated to provide plants with an adaptive advantage.