Allelochemical reactions are chemically mediated interactions between organisms that affect individuals and populations and might influence trophic interactions. Many inter- and intraspecific interactions are, at least in part, regulated by allelochemicals. Predator–prey interactions are influenced by either kairomones allowing prey to chemically detect predators and react by behavioral, morphological or life-history changes, or by alarm cues released from damaged prey evoking behavioral changes in conspecifics. Grazer or herbivores locate food by foraging cues and might be affected negatively by feeding deterrents, inhibitors, or toxins present in food plants or algae. Allelopathy comprises nonexploitative competition among plants and may affect community succession. Allelochemicals are either released by intact or damaged organisms or are located in the tissue. Although some reactions can be unambiguously related to a chemical cue, the structures of many aquatic allelochemicals have not been elucidated. Most known structures have been identified from cyanobacteria, among them many are toxins.