Nereidid polychaetes and indeed many marine invertebrates use chemical signaling via sex pheromones to coordinate their reproductive behaviour. Sex pheromones attract the sexual partner and ensure the coordinated release of gametes by both sexual partners. In the current paper nereidids are used as model organisms to describe the chemical and behavioural basis of chemical communication in marine invertebrates. Structure-activity relationships can be used to chemically classify signals according to their biological function and suitable purification strategies are described. Pheromones in nereidids include diverse molecules such as volatile lipophilic 5-methyl-3-heptanone and 3,5-octadiene-2-one, uric acid and small glutathione derived peptides. A prominent question in chemical signaling is the species specificity of chemical cues and the scale of their distribution in the marine environment. This evolves when one considers the wide overlap in multispecies and multiphyla spawning such as in coral reefs. Behavioural and electrophysiological assays with several nereidid species, confirm the existence of heterospecific activity of body fluids from various phyla. The 'nuptial dance′ reproductive behaviour, as well as the release of gametes, are elicited when individuals are exposed to coelomic fluid of another nereidid species and also from lugworms, starfish, and sea urchins. The function of sex pheromones in nereidid polychaetes and their role in the timing of reproduction in relation to other environmental cues is discussed. Future research will focus on biosynthesis and reception of nereidid pheromones as well as their molecular basis and possible interactions with pollutants such as endocrine disrupters and fuel oil derivatives.