Allelopathy is one of the factors contributing to the formation and maintenance of cyanobacterial and microalgal blooms, which strongly affect coastal marine ecosystems and cause economic problems for aquaculture. Furthermore, some cyanobacteria and microalgae produce harmful compounds which can have negative effects on plants, animals and even humans. Over the past few decades, the world's coastal waters have experienced an increase in the number of harmful algal bloom events. Moreover, it was described that cyanobacterial and microalgal blooms are occurring in more areas than ever before and now this phenomenon is reported regularly. However, despite the seriousness of the problem, reports of the occurrence and consequences of allelopathic effects of cyanobacteria and microalgae in aquatic ecosystems are still insufficient. Therefore, it is important to conduct a more intensive study of this phenomenon.