Phytoplankton species compete with each other for limiting resources, and consequently, they have evolved different strategies to better exploit a resource. Phytoplankton may compete by directly interfering with each other, through e.g., the release of chemicals compounds. Any direct or indirect, harmful or beneficial effect caused by plants, protista (e.g. algae, ciliates), bacteria, or viruses on another, through the production of chemical compounds that escape into the environment, is called allelopathy. Allelopathy is a well-studied phenomenon in terrestrial plant ecology, but its importance in aquatic environments is just starting to be understood. Nevertheless, allelopathy has been suggested to influence phytoplankton competition, succession, and bloom formation or maintenance. It is of special interest to understand if allelopathy contributes to the success of toxic phytoplankton species, since these species cause adverse effects to the environment, public health, aquaculture, and tourism. Thus, the main objectives of my thesis consisted in identifying microalgae species with allelopathic properties, and to investigate if allelopathy is a relevant process in marine waters that can lead to the success of toxic algal species. To investigate these questions, I performed bioassays testing the effect of toxic microalgae species on plankton communities, and on algal monocultures. I also did bioassays to identify factors regulating allelopathic interactions among phytoplankton species. The results presented in this thesis showed that some key factors that affect phytoplankton allelopathy are: the organism involved (donor and target), the degradation of the allelochemicals, the growth phase of the allelopathic species, and stress factors such as nutrient limitation. The allelopathic effect was independent of the production of certain known algal toxins, and it was not caused by bacteria present in the algal cultures. The results also suggested that allelopathy may have an important role in phytoplankton competition. Allelopathic species can decrease the number of competitors by killing other phytoplankton, and also by affecting phytoplankton primary production. Allelopathy may also be used in combination with other strategies, such as mixotrophy, and some allelopathic species are able to affect organisms from different levels of the trophic food web. Thus allelopathic phytoplankton species may have an enhanced competitive advantage. On the other hand, target organisms may have defence mechanisms that will enable them to resist to allelochemicals, such as developmental responses (e.g. encystment). Both the tolerance showed by some target organisms, and the fact that allelochemicals affect selected target organisms suggest that allelopathic interactions can lead to co-evolution of the species involved. Therefore, allelopathy has, undeniably, a great importance within phytoplankton ecology, and should be included among the traditional abiotic and biotic (grazing, exploitation) factors that affect phytoplankton dynamics.