This chapter builds on the understanding of the material properties of homogeneous semiconductors obtained in earlier parts of the book to develop an understanding of pn junctions that join differently doped parts of the same semiconductor, metal-semiconductor contacts (i.e., Schottky junctions), as well as heterojunctions between different semiconductors. A discussion of different approaches toward pn-junction formation leads to a detailed look at the equilibrium band diagram of a pn junction. Schottky junctions are analyzed as a limiting case of a pn junction. The chapter concludes with sections on heterojunctions between semiconductors, both in three dimensions and for atomically thin two-dimensional materials, including the concept of band offsets and an introduction to the methods used for fabricating high-quality heterojunctions.