A circadian clock optimizes many aspects of plant biology relative to the light/dark cycle. One example is the circadian control of primary metabolism and CO 2 fixation in plants that carry out a metabolic adaptation of photosynthesis called CAM (crassulacean acid metabolism). These plants perform primary CO 2 fixation at night using the enzyme phosphoenolpyruvate carboxylase and exhibit a robust rhythm of CO 2 fixation under constant conditions. Transcriptomic analysis has revealed that many genes encoding enzymes in primary metabolic pathways such as glycolysis and starch metabolism are under the control of the circadian clock in CAM plants. These transcript changes are accompanied by changes in metabolite levels associated with flux through these pathways. The molecular basis for the circadian control of CAM remains to be elucidated. Current research is focusing on the identity of the CAM central oscillator and the output pathway that links the central oscillator to the control of plant metabolism.