The formation and structure of various faces of GaN, including adsorbed layers of Al, In, or H, are discussed. The primary preparation method is plasma-assisted molecular beam epitaxy although a comparison of results from metal-organic vapor phase epitaxy will also be presented. Experimental results from scanning tunneling microscopy allow us to broadly determine the characteristics of the surface structures, and first principles theory is then used to determine the precise atomic arrangements. For the case of surfaces grown by vapor phase epitaxy spectroscopic ellipsometry is used to identify the relevant surface phases. In contrast to other semiconductor surfaces, a central feature of many GaN reconstructions is their tendency to form metallic overlayers of metal (Ga, In, or Al) atoms. The terminating layers of metal atoms also leads to novel aspects of the surface kinetics - N atoms are predicted to diffuse easily between the metal layers, thus yielding enhanced surface diffusivity for those surfaces which are terminated by more than one layer of metal atoms. @FootnoteText@ Work performed with Y. Dong, C. D. Lee, H. Chen, A. R. Smith (CMU); J. E. Northrup (PARC); J. Neugebauer (FHI, Berlin); C. Cobet, T. Schmidtling, M. Drago, N. Wollschlaeger, N. Esser, W. Richter (TU, Berlin); and supported by NSF and ONR.