Safe drinking-water is of paramount importance for human health. Among the hazards considered in the Guidelines for Drinking Water Quality, infectious microorganisms are the most significant causes of mortality on a global scale, causing a substantial burden of disease via diarrhoeal illnesses such as cholera, cryptosporidiosis or retroviral enteritis. Cyanobacteria have been present in natural ecosystems since the Precambrian Era, some 2 billion years ago, and the production of cyanotoxins is probably an equally ancient characteristic. This health hazard most probably will gain growing importance as cyanobacterial blooms are expected to increase at the scale at which eutrophication is expected to increasingly occur in many regions of the world. Epidemiological studies have looked for chronic effects in human populations exposed to toxic cyanobacteria, and indeed, a number of studies since the mid-19th century associate symptoms with cyanotoxin exposure. The chapter also presents an overview of the key concepts discussed in this book.