In this chapter the one-electron energy band theories for the crystalline solids are presented. The importance of energy band theories for a crystalline solid is due to the fact that many important physical and optical properties of a solid can be readily explained using its energy band structure. In general, the energy band structure of a solid can be constructed by solving the one-electron Schrödinger equation for electrons in a crystalline solid that contains a large number of interacting electrons and atoms. To simplify the difficult task of solving the Schrödinger equation for the many-body problems in a crystal, the effects that arise from the motion of atomic nuclei must be neglected (i.e., it is assumed that the nuclei are at rest in the equilibrium positions at each lattice site). Under this condition, the nuclear coordinates enter the problem only as a constant parameter. However, even though the problem is confined as a purely electronic one, there are still the many-electron problems in the system that cannot be solved explicitly. Therefore, it is necessary to apply additional approximations in solving the Schrödinger equation for electrons in a crystalline solid.