This article is a review of the various methods in use, or under development, to calibrate space-multispectral-imaging-systems in the solar-reflective range. We introduce the subject by distinguishing between absolute and relative calibration, briefly discussing the use of scene and sensor models, and describing five calibration desiderata. We then briefly describe the different types of existing methods, highlighting their advantages and disadvantages, making the distinction between: preflight, onboard, and vicarious calibration. The different types of radiometric calibration: absolute, multitemporal, interband, and intersensor are mentioned with their related constraints. Finally, recommendations are made on how to improve these methods so that the scientific community may obtain remote-sensing data of the highest quality.