Forty-percent of mammal species are rodents, and they are found in vast numbers on all continents other than Antarctica. Common rodents include mice, rats, squirrels, chipmunks, gophers, porcupines, beavers, hamsters, gerbils, and guinea pigs. Rodents have sharp incisors that they use to gnaw wood, break into food, and bite predators. Most eat seeds or plants, though some have more varied diets. They have historically been pests, eating human seed stores and spreading disease.

Rodents evolved some time around the end of the Cretaceous period c 65 million years ago.

In terms of number of species — although not necessarily in terms of number of organisms (population) or biomass — rodents make up the largest order of mammals, with over 40 percent of mammalian species belonging to the order.[1] Their success is probably due to their small size, short breeding cycle, and ability to gnaw and eat a wide variety of foods.(Lambert, 2000)

There are about 2,277 species of rodents, about 42% of all mammal species. Rodents are found in vast numbers on all continents except Antarctica, most islands, and in all habitats except for oceans. They are the only placental order other than bats (Chiroptera) and sea lions (Otariidae) to reach Australia without human introduction.