This review is devoted to analyzing the global occurrence of cyanobacteria in water ecosystems, the possible causes of this phenomenon, and its consequences. In recent decades, cyanobacteria have been rapidly expanding in waterbodies all over the world. This expansion is accompanied by water pollution with dangerous cyanotoxin metabolites that represents a significant threat to humans, animals, and the environment. Purifying water of cyanobacteria is a serious problem, because it is necessary to eliminate toxins and the unpleasant taste and odor of drinking water, as well as fight the biocorrosion caused by cyanobacterial fouling. Cyanobacterial blooms concern not only issues related to the water supply, but also to fishing, the recreational use of waterbodies, and tourism. Global warming and climate change, the increasing eutrophication of natural waters, and anthropogenic pollution, as well as the unique physiological characteristics of cyanobacteria and their ability to adapt to a variety of environmental conditions, including extreme environments, are among the main factors contributing to the expansion of cyanobacteria.