All vipers fall under the family Viperidae. However, it contains two sub-families that distinguish between what we call “True Vipers” and “Pit Vipers.” The distinction is whether or not a viper has heat-sensing organs, which we call “pits.” Pit Vipers have these heat-sensing organs and belong to the subfamily Crotalinae. True Vipers, on the other hand, lack these heat-sensing organs and belong to the subfamily Viperinae. Some genera under Crotalinae include Agkistrodon, Bothrops, Crotalus and Trimeresurus. Some genera under Viperinae include Atheris, Bitis, Cerastes, and Echis.