The skeleton of sharks is to a large extend made out of cartilage, unlike the rest of the vertebrates skeleton, which are made out of bone. Cartilage is a type of connective tissue composed of cells called chondrocytes suspended in a matrix of protein. The parts of the skeleton that are in need of extra support e.g. jaws, vertebrae or gill arches, are build up by calcified cartilage. The calcified cartilage is harder than normal cartilage due to deposits of calcium salt. Often, the appearance of calcified cartilage and bone is very similar, and differences are apparent only after the tissues have been specially treated and examined with a microscope. The main difference between bone and cartilage is that bone is composed of osteocytes instead of chondrocytes.

The cartilaginous skeleton of the modern sharks is something that probably has evolved from a bony skeleton, which means that the previous ancestor of today's sharks had a skeleton made out of bone. There is no reason to believe that having a cartilaginous skeleton is disadvantageous. On the contrary, cartilage is less dense and more elastic than bone, providing advantages of buoyancy and flexibility.

Because cartilage and other soft parts decay rapidly, it is rare to find complete or even nearly complete fossil remains of sharks. This is one reason why evolutionary relationships of sharks are so difficult to trace. Often teeth, scales, spines or vertebrae are the only parts remained in fossils, and from these parts alone, it is very hard to figure out what the entire shark looked like.