Hylids (treefrogs) are one of the largest families of frogs. There are about 37-39 genera. These are arranged in four subfamilies, the first three of which are distinctive: Pelodryadinae, Phyllomedusinae, Hemiphractinae, and Hylinae.

Among the most bizarre hyline frogs are certain casque-headed genera, such as Triprion and Trachycephalus, in which the skull bones are elaborated into a solid helmet. Certain casque-headed species use their bony heads to block the entrances to their burrows and reduce evaporative water loss.