Notwithstanding the broad philosophical implications of Niels Bohr's thought reaching far beyond the confines of physics, quantum theory was from the beginning, and remains to be, the field of primary importance with which his idea of complementarity is concerned. Understanding of his thought therefore requires at least a brief survey of quantum physics as it historically developed in the early twentieth century. In this opening chapter, I wish accordingly to offer a sketch of the development of quantum theory – including the central role played by Bohr – within which his path toward the idea of complementarity is situated.