Theory of low-energy electron diffraction (LEED) is explained. The discussion is concentrated to the methods of diffraction-intensity calculation. Particular conditions encountered in applying the dynamical theory to LEED are discussed. The three methods of analysis used at present i, e. Bethe's theory, the multiple-scattering theory and multislice theory, are explained. Particularly, the new concepts introduced from band theory, i, e. the muffin-tin model, the pseudopotential, etc. are explained.