Abstract:Allelopathy may be one of factors contributing to the formation and/or maintenance of harmful algal blooms（HABs）, which are a serious ecological and socioeconomic problem facing the planet today. Because phytoplankton allelochemicals have the potential to deal with HABs, more attention are paid to the alleolpathy in phytoplankton. Progress and achievements are geting in this field and more and more allelochemicals are successfully purified and identified in cyanobacteria, dinoflagellate, green alga and diatoms. Most studies have observed and described the phenomenon of the allelopathy, but further detailed studies on the mode of action of allelochemicals are limited. The allelochemicals are capable of interrupting or stimulating many essential functions of prokaryotic and eukaryotic cells, cause the death of other algae, bacteria, viruses, and fungi. The allelochemical producer thereby have an adaptive advantage over other species. Authors focus on allelopathic interactions in marine and freshwater phytoplankton. Discussf the characteristics of allelopathy in phytoplankton, the basic approaches on allelopathy research, the chemical properties of allelochemicals and the factors affecting their production, particularly discuss the mode of action of allelochemicals and the defence mechanism to resist allelochemicals.