The exact features of the band structure of semiconductor materials play a crucial role in determining their electronic and optoelectronic properties and their usefulness for devices such as transistors, microwave oscillators, lasers, light emitting diodes, and detectors. This chapter examines some of the main features of the electronic band structure of bulk semiconductors, including the influence of defect atoms on the band structure. It shows how the k • p method can be used to understand common features and trends in the electronic properties, including the effects of alloying and hydrostatic pressure. The chapter introduces the concept of effective mass and the concept of positively-charged holes. It considers the effects of impurity atoms and what happens when the crystalline periodicity breaks down, giving an amorphous semiconductor. Some impurity atoms can be positively beneficial, introducing shallow defect levels in the band gap, close to the band edges.