In this chapter, the framework for describing the behavior of electrons in semiconductors is developed. An introduction of basic elements of quantum mechanics is followed by a discussion of Bloch’s theorem for electrons in periodic potentials, culminating in the concept of the dispersion relation of electrons (or band structure) in reciprocal space. The reader is then familiarized with the main attributes of the band structure of semiconductors, including the notion of the effective mass and the difference between direct and indirect gap materials, followed by examples of band structures of technologically important semiconductors. The simplified representation of the electronic structure by the band diagram and its connection with the full dispersion relation is presented. The chapter concludes with a survey of the methods used in band structure calculations and measurements and a discussion of the thickness-dependent band structure of layered (van der Waals) semiconductors.