Animals that successfully use vision in the deep-sea environment possess some unusual adaptations to maximize their sensitivity in this very low light environment. In spite of their smaller size, deep-sea crustacean photoreceptors are approximately 80 times more sensitive than human photoreceptors, enabling them to be able to detect downwelling irradiance at a depth of 950 m in clear ocean water. This high sensitivity is accomplished through a variety of optical and physiological mechanisms to improve photon capture. These physiological mechanisms include shifts in the absorption maxima of visual pigments as well as monochromacy, and neural adaptations such as an increase the integration time of the photoreceptor. However, another significant source of light in the deep-sea is bioluminescence, and some crustaceans possess visual mechanisms which are clearly adaptations for looking at bioluminescent point sources rather than downwelling irradiance. The current state of knowledge of visual adaptations in deep-sea crustaceans will be summarized, and the relevance of these adaptations to what is known about their life history and ecology will be discussed.