Seeing is important for most species and it has been a key selective advantage throughout evolution. Consequently, there is a remarkable diversity among types of eyes. Animals have converged on eight optical solutions for collecting and focusing light; in contrast, all eyes share the same molecular strategy for absorbing photons. Recent studies have identified similarities in the genetic information that is used in the development of eyes, leading to the hypothesis that distinctly different eye types might have had a monophyletic origin. Across many species, there is a remarkable continuity of the developmental genes that participate in the construction of similar--but not necessarily homologous--eyes.