The genus Ansonia (stream toads) currently comprises roughly three dozen recognized species distributed from Myanmar through Sundaland to the Philippines, and many additional species are likely yet to be discovered. Members of this genus typically have relatively long, slender limbs and narrow bodies. They are small to medium-sized toads, usually brownish in coloration, with dry, keratinized, warty skin that may bear small spines. Unlike many other bufonids, species of Ansonia lack distinct parotoid glands. Nevertheless, they produce potent skin toxins that can, for example, readily kill other frogs when temporarily housed in the same container.

Species of Ansonia inhabit forest streams, and their tadpoles are rheophilous—that is, adapted to life in flowing water. Tadpoles of different species show varying degrees of specialization in relation to current velocity. On Borneo, Ansoniatadpoles often occur together with those of the ranid genus Meristogenys, sometimes sharing the same submerged or periodically overflowed rocks. They are active both during the day and at night, but tend to be more exposed after dark, when they feed on the surfaces of rocks washed by the current.

The taxonomy of Ansonia remains in flux, and further revisions can be expected. Some species are difficult to distinguish morphologically, and as a result, the true diversity of the genus is still not fully understood.