This chapter focuses on the inducible defences evoked by chemical cues associated with predation. It describes numerous examples of the ways in which Ostariophysan fishes use chemical alarm cues to assess predation risk. The chapter examines the essential role of Ostariophysan alarm cues in assessing predation risk. Ostariophysans make excellent test subjects for examining how public information can be used to assess predation risk. The majority of studies examining anti-predator responses upon exposure to Ostariophysan alarm cues are conducted under controlled laboratory conditions. Ostariophysans clearly benefit through direct responses to alarm cues. It is well established that the alarm pheromone of fathead minnows and other Ostariophysan fishes can pass through the digestive system of fishes and can still be recognized. At the ultimate level, more information is needed about how the aquatic shapes responses to chemical alarm cues by Ostariophysans.