How many different kinds of animals are there?

Experts estimate that more than 100 million different kinds of animals have been identified in the world. There may be millions more, particularly insect species, that have not yet been identified or discovered by scientists. Hundreds of years ago scientists began dividing the animal kingdom into categories based on certain characteristics like body type, ways of reproducing, and what the animals can do (fly, swim, walk on two legs, and so on). The animal kingdom, and every other kingdom as well, is divided and subdivided into numerous other categories.