The brain's biological clock located in the suprachiasmatic nucleus (SCN) generates circadian rhythms of physiology and behaviour of approximately 24 hours. The clock needs, however, like a watch that runs too fast or too slow, daily adjustment and the most important stimulus for this adjustment is the environmental light/dark cycle, a process know as photoentrainment. It is well established that the eye contains a separate anatomical and functional system mediating light information to the clock. Until recently, the photopigment responsible for light entrainment of the circadian system has been elusive but recent studies have provided evidence that melanopsin, a recently identified opsin, could be the circadian photopigment. This conclusion is based on the observation that melanopsin is expressed exclusively in retinal ganglion cells projecting to the SCN, a projection known as the retinohypothalamic tract (RHT) and that these ganglion cells are intrinsically photosensitive. Melanopsin is present in the plasma membrane of soma, dendrites and axons forming an extensive photoreceptive network in the entire retina. Although these findings make melanopsin a strong candidate as a circadian photopigment, a number of functional experiments are needed before the role of melanopsin is finally proven.