Sensory systems provide crucial information about an organism's external environment and, thus, are often subject to strong natural selection. Because of the large variation in the intensity and spectral quality of light in aquatic environments, studies of sensory adaptation have focused on the visual systems of fish for over a half a century. Recently, the molecular genetic mechanisms that determine the spectral sensitivity of visual pigments have been characterized in several fishes including zebrafish, guppies, medaka, killifish, bream, and cichlids. The results of these studies suggest that teleost fish have incredibly diverse visual systems. In this paper, we review the role that opsin duplication and differential gene expression have played in the diversification of visual pigments. We compare our findings in cichlids to five other taxonomic groups and highlight the ways that their similarities and differences may provide new insights into the molecular genetic basis of sensory adaptation and diversification.