All of the most commonly encountered genera of cyanobacteria which form blooms and scums in fresh-brackish- and marine waters include members capable of producing potent toxins. Poisoning of vertebrate and invertebrate animals following the ingestion of cyanobacterial bloom/scum material have been widely reported for many years and recognition of the adverse effects of cyanobacterial blooms and their toxins is increasing. This review considers the occurrence of toxic cyanobacterial populations and properties of the toxins themselves, of which at least 60 are now recognised. When rightfully regarded as microbial secondary metabolites, a range of possible functions for cyanobacterial toxins is presented. Whether cyanobacterial toxins contribute to the ability of cyanobacteria to dominate many eutrophic waterbodies is unknown although understanding of the occurrence of the toxins in aquatic environments and their actions at the molecular level and with whole organisms in laboratory studies indicates that this is possible.