Freshwater cyanobacteria and algae are responsible for the formation of a wide spectrum of different odorous compounds that are frequently observed in natural waters and are the causes consumer complaints. The physiology and biochemistry of odorous compounds that are permanently in the cells or excreted into the environment are here described, as are the odorous compounds that are generated only after disintegration of the cells and are produced by the initiation of lipoxygenase and carotene oxygenase reactions.