Calibration requirements for remote sensors designed to view the ocean are significantly more demanding than those for land-viewing sensors. This is due to the fact that the albedo of the ocean is very low and the atmosphere contributes a major portion (80–100%) of the radiance measured at the sensor. Furthermore, the atmospheric contribution in some spectral regions is principally due to Rayleigh scattering. Removal of this component from the imagery requires the extraterrestrial solar irradiance. Thus, in addition to precise sensor calibration, precise intercalibration with the extraterrestrial solar irradiance is required as well. In this paper the calibration requirements for ocean-viewing sensors are outlined, and the present methods of effecting such calibration are described in detail. For future instruments it is suggested that provision be made for the sensor to view solar irradiance in diffuse reflection and that the moon be used as a source of diffuse light for monitoring the sensor stability.