Melanin, a well-known macromolecule that is widely distributed in many parts of nature. Polydopamine is also a major pigment of naturally occurring melanin (eumelanin). As a kind of macromolecule containing a constitutional unit with catechol, amine, and imine. Melanin can scavenge radical, absorb ultraviolet rays and exhibits excellent photoprotective, adhesive properties, and photothermal conversion ability. Various substrates could be successfully functionalized with melanin. The functional groups on melanin can serve as both the starting points for covalent modification with desired molecules, so it is easy to modify the surface of diverse materials. There are two methods to prepare melanin models: natural melanin model and synthetic melaninlike model. The natural melanin model has been obtained from a biological source. The synthetic melaninlike model is obtained by chemical oxidation of dopamine or enzymatic oxidation of precursor molecules. Both natural melanin and melaninlike pigment show similar physical and chemical properties. In this paper, the structure and the mechanism of melanin formation, and the application of melanin in composites material are reviewed.