This chapter considers Bohr's argument concerning, in his phrase, "the epistemological lesson of quantum mechanics," an argument advanced under the general heading of complementarity, the term that initially referred to a concept and a principle but that eventually came to designate Bohr's interpretation of quantum phenomena and quantum mechanics. While centered on Bohr's ultimate, RWR-principle-based, interpretation, the chapter will address the development of Bohr's thinking, in part under the impact of his debate with Einstein, leading Bohr to this interpretation. Sect. 3.1 offers a general introduction. Sects. 3.2 and 3.3 consider the concept and principle of complementarity. Sect. 3.4 discusses the EPR experiment and what I shall call "the EPR complementarity," which grounded Bohr's analysis of the experiment in his reply to EPR's paper. Sect. 3.5 discusses Bohr's ultimate, RWR-principle-based, interpretation. Sect. 3.6 considers Bohr's view of quantum probability and statistics.