Although it has been recognised for many years that pheromones play important roles in fish reproduction, only recently has progress in identifying three chemical classes of reproductive pheromones (bile acids, sex steroids, and F prostaglandins) in a variety of fish provided clear insight into both the complexity and diversity of their functions. Although bile acids are produced, released, and detected by many fishes, only in the sea lamprey is their function understood. Here, different sets of bile acids function as attractants for migratory adults and as a male sex pheromone, raising the possibility that this class of compounds might also have multiple, endocrine-associated functions in many other species. Better understood are the sex steroid and prostaglandin pheromones (hormonal pheromones), which are widespread if not ubiquitous among the bony fishes, where they are detected with great sensitivity and specificity, and induce potent and specific behavioural and physiological effects in such major taxa as carps and salmonids. Because these hormonal pheromone systems chemically link an individual's endocrine system with the nervous systems of conspecifics, they challenge classical concepts that sex hormone actions are restricted to the individual. Indeed, the interrelationships among hormones and pheromones appear so pervasive that it now seems prudent to assume that many aspects of fish reproductive endocrine function can be understood only in the context of the social group. Despite these exciting advances, major aspects of reproductive pheromone function remain almost unexplored. Remarkably little is known about the metabolism and release of bile acids, sex steroids, and, particularly, prostaglandins. Even less is known about how these cues might function as mixtures that could contribute to species specific function; progress in these latter areas is essential if we are to determine the true nature of fish reproductive pheromones.