The potential of carbon monoxide to act as a therapeutic agent is now well-established. Controlled delivery of CO is best achieved using 'CORMs': molecules which release known amounts of carbon monoxide in response to a stimulus. Metal carbonyl complexes will release CO if irradiated with ultraviolet light, but it is only in the past five years that development of true 'photoCORMs' has been explored. Recent exciting developments in this area now show that design of photoCORMs operating well into the visible region is achievable. In this Perspective, we examine the growth of photoCORMs from their origins in the photophysics of metal carbonyls to the latest visible-light agents.