This book offers a new perspective on Niels Bohr's interpretation of quantum mechanics as complementarity, and on the relationships between physics and philosophy in Bohr's work. The importance of quantum field theory for Bohr's thinking has not been adequately addressed in the literature on Bohr. This book provides clarification of Bohr's writings (which usually pose problems of reading), and an analysis of the role of quantum field theory in Bohr's thinking.