The aquatic environment is well suited for the transmission of chemical information. Aquatic animals have evolved highly sensitive receptors for detecting these cues. Here, I review behavioural evidence for the use of chemical cues by aquatic animals for the assessment of predation risk. Chemical cues are released during detection, attack, capture and ingestion of prey. The nature of the cue released depends on the stage of the predation sequence in which cues are released. Predator odours, disturbance pheromones, injury–released chemical cues and dietary cues all convey chemical information to prey. Prey use these cues to minimize their probability of being taken on to the next stage of the sequence. The evolution of specialized epidermal alarm substance cells in fishes in the superorder Ostariophysi represent an amplification of this general phenomenon. These cells carry a significant metabolic cost. The cost is offset by the fitness benefit of the chemical attraction of predators. Attempts of piracy by secondary predators interrupt predation events allowing prey an opportunity for escape. In conclusion, chemical cues are widely used by aquatic prey for risk assessment and this has resulted in the evolution of specialized structures among some taxa.