All animals (and plants) fall into a modern classification—a system first developed by Swedish naturalist Carolus Linnaeus (Carl von Linné, 1707–1778). In the standard Linnean system of animal classification, the hierarchy to dinosaurs is as follows: Animalia (kingdom); Chordata (phylum); Reptilia (class); and Dinosauria (infraclass).

There are even more divisions, and the list seems to get longer and longer every time another dinosaur fossil is found. But in general, they are often divided into two main groups, based historically on their hipbone structure. Those with hips that had the two lower bones pointing in opposite directions, with the pubis bone pointing forward are called saurischian, or lizard-hipped dinosaurs. Those with hips that had the two lower bones lying together behind the back legs, and the pubis bone pointing backward are called ornithischian, or bird-hipped dinosaurs. Tyrannosaurus rex is an example of a lizard-hipped dinosaur; while the Iguanodon is an example of a bird-hipped dinosaur.