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AbstractThe chemical ecology of marine eukaryotic microalgae is the study of biological and chemical interactions between these organisms in their aquatic environment. Chemically mediated communication and cues are responsible for conditioning the milieu for growth and cellular defence, potentially affecting processes as diverse as inter- and intraspecific competition, sexual reproduction, sinking and swimming behaviour, solar radiation protection, predation, and nutrient acquisition and sequestration. Although the biosynthesis of the secondary metabolites that elicit these responses may confer a significant (if not unique) selective advantage upon the producer, both in terms of evolutionary history and over seasonal time-scales in marine food webs, it is difficult to demonstrate the functional mechanisms with certainty. This has engendered a fruitful, albeit hypothetical, discussion on the ecoevolutionary importance of these secondary metabolites. In comparison to the large number of studies on the chemical ecology of terrestrial plants, bacteria and fungi, and invertebrates from the marine environments, the marine eukaryotic microalgae are underrepresented. Most studies on the secondary metabolites of eukaryotic microalgae have focussed only on bioactivity and structural elucidation, with a view towards potential pharmaceuticals or other commercial and regulatory purposes. Nevertheless, the environmental and human health consequences of harmful algal blooms, many of which exert their deleterious effects by means of potent phycotoxins, have provided impetus to conduct ecologically relevant studies. In this review, the theoretical and practical bases of chemical ecological interactions are presented for marine eukaryotic microalgae. Definitions of the types of interactions mediated by these bioactive substances (pheromones, kairomones, allomones, etc.) are accompanied by examples from the eukaryotic microalgal literature. The Focus of this review is primarily upon the known and well-characterized phycotoxins, which are described in terms of both structural and functional relationships (where known). By a selective synthesis of these relationships, the eventual objective of understanding the role of these fascinating and diverse compounds in marine ecosystems may be achieved.