This book discusses toxic Microcystis and the toxins from various viewpoints such as classification, cultivation, occurrences in lakes, and relations to zooplankton. The text presents new information on the chemistry, analytical chemistry, toxicology, molecular modeling, and liver tumor promotion of the toxins. Microcystis species are described in relation to morphological features, allozyme genotype, and toxin content. Seasonal changes of Microcystis population are described with special references to toxic species and composition of the toxins. Chemical characteristics of microcystins are reviewed and the process for identification of microcystins is described.