The cyanobacterium Synechococcus elongatus PCC 7942 has been established as the model system for studying the molecular mechanisms of the cir-cadian clock in cyanobacteria. This chapter mainly focuses on other cyanobacteria, such as SynechocystisL sp. strain PCC 6803, Thermosynechococcus elongatus and the genera Trichodesmium and Prochlorococcus. Here, we describe the research background, current status, possible problems and perspectives for studying circa-dian rhythms for each species/group and we summarize the related works of other cyanobacteria and plastids.