Polymers are substances that consist of very large molecules, each composed of numerous repeating subunits, or monomers. This structure results in unique and very useful physical properties including toughness, high elasticity, and a tendency to form amorphous and semicrystalline structures rather than crystals. An extensive variety of polymers exists, ranging from synthetic plastics, such as polycarbonate and polystyrene, to natural biopolymers, such as proteins and DNA, that are fundamental to all life.