Cell division cycles and circadian rhythms are major periodic phenomena in organisms. Circadian oscillators control biochemical, physiological, and behavioral events in a wide range of living systems including almost all eukaryotes that have been tested and some prokaryotes-in particular, the cyanobacteria. Gating of cell division is one of the key processes that has been reported to be regulated by circadian clocks in many organisms. We survey studies of the circadian control of cell division in eukaryotic microorganisms and introduce recent progress on understanding the interaction between circadian rhythms and cell division cycles in cyanobacteria.