Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a special type of ganglion cell that express the photopigment melanopsin and autonomously respond to light in the mammalian retina. Nearly two decades since their discovery, ipRGCs have been broadly investigated for their basic morphologic features, cell classification, physiologic properties, and functional significance. It is well established that ipRGCs are predominantly responsible for non-image-forming vision, such as circadian rhythm entrainment, pupillary contraction, mood, and sleep regulation.