Toxic cyanobacteria are increasingly recognised as a potential hazard in water for human use. The recent availability of methods for quantitative screening is leading to a surge of research on their occurrence. This book presents a novel compilation of extensive screening results showing the cyanotoxin levels that may be expected when certain taxa dominate. These results indicate cyanotoxins to be the most widespread among the chemicals of concern in water used for drinking and recreation. It further combines field data with results from laboratory culture experiments to suggest a unifying view of how environmental factors control the cyanotoxin levels in natural waters. A practical section is dedicated to the exposure of humans through drinking-water and recreation.