Abstract: Aquatic mammals had to adapt to visual environments that are very different from those encountered by terrestrial mammals. Herein, we review the current knowledge on the colour-vision capabilities of aquatic mammals and discuss the puzzles that emerge from those studies. The common pattern in terrestrial mammals is dichromatic colour vision based on two spectral types of cone photoreceptors in the retina (commonly green and blue cones). Behavioural studies in a few pinnipeds and one dolphin species suggest a similar type of dichromatic colour vision in marine mammals. In contrast, recent immunocytochemical, physiological, and molecular genetic studies of the cone visual pigments in a larger sample of species show that whales and seals generally lack the blue cones and presumably are green cone monochromats. This challenges the behavioural findings, because cone monochromacy usually is tantamount to colour blindness. Furthermore, a loss of the blue cones seems a rather inadequate adaptation to the blue-dominated underwater light field in the open ocean. Other aquatic and amphibious mammals (sirenians, otters, hippopotamuses and polar bear) appear to show the more common pattern of cone dichromacy. The present review summarizes available data on the various aquatic mammalian taxa, assesses the reliability of these data, and discusses the potential adaptive pressures involved in blue cone loss. It is suggested that blue cones were lost in an early, 'coastal' period of cetacean and pinniped evolution; many coastal waters preferentially absorb blue light and constitute a long-wavelength-dominated environment. Residual colour vision in these cone monochromats could be achieved in mesopic lighting conditions by exploiting the signal differences between the remaining green cones and the rods.