Castorids live in small family groups that each occupy a specific territory, based around a lodge and dam constructed from sticks and mud.

[1] They have powerful incisors and the typical rodent dental formula: The earliest castorids belong to the genus Agnotocastor, known from the late Eocene and Oligocene of North America and Asia.

[3] Their teeth were not well suited to gnawing wood, suggesting this habit evolved at a later point, but they do appear adapted to semiaquatic living.

Giant forms evolved in the Pleistocene, including Trogontherium in Europe, and Castoroides in North America.

Within the family, Castorinae and Castoroidinae are sister taxa; they share a more recent common ancestor with each other than with members of the other two subfamilies.