This paper represents a comprehensive review of the available data on toxic algal blooms in freshwater ecosystems, with emphasis on the appearance of cyanobacterial blooms in Kopački rit floodplain. The results indicate that cyanobacteria often bloom extensively in eutrophic freshwater ecosystems, especially in shallow lakes such as lakes in the floodplain wetland of Kopački rit. Their development is determined by many factors, among which the most important are nutrient concentrations, temperature, and watertransparency. Lake morphometry, the structure of the food chain, as well as conditions of mixing of water and sediment are also considered important. Many cyanobacterial species of the genera Anabaena, Aphanizomenon, Microcystis, Cylindrospermopsis and Planktothrix, which are appearing in the waters of Kopački rit, contain different types of toxins (microcystin, anatoxin-a, saxitoxin, cylindrospermopsin).During their development, the presence of high levels of cyanotoxins may occur, and this can lead to poisoning and the increased mortality of the living world (plants, invertebrates, fish and other animals) that live in the water or drink water. Particular attention in recent decades is given to monitoring the spread of tropical invasive species Cylindrospermopsisraciborskii in European waters.