In mammals, the intrinsic circadian clock regulates various physiological rhythms, including sleep-wake cycles. These circadian rhythms can be entrained to daily 24-hour light-dark cycles by virtue of photoreception within the retina. The photo-entrainment of circadian rhythms is predominantly mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). In this article, we review the mechanisms of action of ipRGC photoreception, the retinal circuit involving ipRGCs, and the heterogeneity of ipRGCs in mice.