Th e infrared (IR) receptors in the pit organ of crotaline snakes are very sensitive to temperature. Th e sensitivity to IR radiation is much greater in crotaline snakes than in boid snakes because they have a thermosensitive membrane suspended in a pair of pits that comprise the pit organ. Th e vasculature of the pit membrane, which is located near IR-sensitive terminal nerve masses, the IR receptors, supplies the blood necessary to provide cooling and the energy and oxygen that the IR receptors require. Th e ophthalmic and maxillary branches of the trigeminal nerve innervate the pit membrane. In crotaline snakes, the trigeminal ganglion (TG) is divided into the ophthalmic and maxillomandibular ganglia; a prominent septum further separates the two divisions of the maxillomandibular ganglion. Th e TG neurons in the ophthalmic ganglion and the maxillary division of the maxillomandibular ganglion relay IR sensation to the brain. Th is article reviews the IR-sensitive pit organ and trigeminal sensory system structures in crotaline snakes.