The tree frogs are a huge family of nearly 700 species in 40 or so living genera. Various genera are constantly being reclassified as members of the family, or are moved out of the group – it is difficult to say with certainty what the precise numbers are at any point in time. In recent years, newly discovered frog species have frequently been deemed members of the hylid family. Their arboreal nature and presence high within the canopy of many tropical and subtropical forests is probably responsible for the constant spate of new discoveries.

Tree frogs are unique in having somewhat disc-shaped pads on their toes, which are composed of interlocking columnar cells that act very much as Velcro does, enabling them to stick to rough surfaces upside down and sideways or to dangle by a single limb from a branch high above the ground like a circus acrobat. On smooth surfaces, mucus-producing cells produce a sticky substance that acts as an adhesive, also enabling them to walk on walls, even when the walls are as smooth as glass.

Although the majority of hylids lay aquatic eggs and have a free living tadpole stage, a number of genera have specialized life histories. Notable among these are frogs of the genus Gastrotheca or the marsupial frogs, which are found in the Andean region of South America, in Ecuador and Colombia on the Pacific coast and from Venezuela to eastern Brazil and northern Argentina on the Atlantic sides, as well as in Panama. The females of these frogs have dorso-lateral pouches into which they push their eggs following fertilization. Here they develop into either tadpoles or fully formed froglets (depending on the species). When they are ready to emerge, the female uses the toes of her rear limbs to stretch open the slits of her pouch to allow them to escape. If she is carrying tadpoles, she releases them into the water. Species where the larvae develop inside the pouch are released either in shallow water or on land.

The gorgeous poster frog of the save the rainforest movement, the Red Eyed Leaf Frog or Tree Frog, is a member of this family. The tree frogs are widely distributed throughout North, Central and South America, Europe ( but not the British Isles) in a narrow corridor eastward through the former USSR into China and Japan, on the Island of Papua New Guinea and through a large area of Australia, as well as on many islands in the Caribbean and elsewhere. They are absent from Africa, India, and Southeast Asia.