What do animals know about their world, and how do they know it? Indeed, all animals experience the world through their perceptions, so their worlds are literally defined by the sensory information available. I am particularly interested in understanding the tremendous diversity of sensory systems across the animal kingdom. The breadth of sensory systems found in fishes (the most diverse group of animals with backbones) offers countless opportunities to examine the results of natural evolutionary experiments. What can some species perceive that others cannot, and what specific features of their sensory apparatus are responsible for that difference in performance? We use a variety of techniques to probe the relationship between nervous system structure and sensory function, ranging from studies of peripheral sensory structures and the organization of sensory processing in the brain, to behavioral and physiological studies of sensory abilities. We focus specifically on the evolution of enhanced hearing in Malagasy Cichlids, the functional relationship of hearing and lateral line senses in multiple groups of fishes, and the evolution of electric communication and jamming avoidance in South American electric fishes.