Circadian regulation is essential for optimum plant performance. In addition to loops and cascades of transcription and translation, the plant circadian clock and its associated signal transduction networks incorporate many post-translational mechanisms. Phosphorylation is a common feature of signal transduction and gene regulation. In this opinion article, we illustrate how phosphorylation events are positioned within the entrainment, functioning, and regulation of the circadian timing system. Phosphorylation regulates protein stability, protein-protein interactions and protein-DNA interactions within the core oscillator. We suggest that phosphorylation provides a potential mechanism for the distribution of circadian timing information within the cell and for the integration of circadian timing information with other signaling pathways.