Castorids are medium sized mammals, although large compared with most other rodents. They are semi aquatic, with sleek bodies and webbed hind feet, and are more agile in the water than on land. Their tails are flattened and scaly, adaptations that help them maneuver in the water.

Castorids live in small family groups that each occupy a specific territory, based around a lodge and dam constructed from sticks and mud. They are herbivores, feeding on leaves and grasses in the summer, and woody plants such as willow in the winter.  They have powerful incisors and the typical rodent dental pattern.