The circadian clock is an internal time-keeping system that generates c. 24 h circadian rhythms. These rhythms are thought to align the phase of biological processes with the time of day in the environment. Three key properties of circadian rhythms are: free running of the rhythm without external cues, entrainment to external cycles, and temperature compensation of the period. These properties are widely conserved across organisms. Circadian rhythms in plants are thought to be generated by a transcriptional/translational feedback loop (TTFL) formed from core clock genes and proteins (Nohales & Kay, 2016). These circadian oscillations, and the circadian clock, can function even in single plant cells. Temporal information derived at the single-cell level is thought to be shared among cells, tissues, and organs, resulting in coordinated rhythms at the organismal level. In other words, circadian oscillators can produce physiological rhythms that have hierarchical organizational architectures. The mechanisms of the circadian clock have been studied extensively using the model plant Arabidopsis thaliana (Arabidopsis). However, there have been exciting developments in the investigation of circadian regulation in divergent plants ranging from green algae to crops. The knowledge obtained from various plant species is advancing understanding of the similarities and diversities of circadian clock mechanisms in the green lineage.