Rodentia is an order of mammals also known as rodents, characterised by two continuously-growing incisors in the upper and lower jaws which must be kept short by gnawing.

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

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 to reach Australia without human introduction.