This volume, part of the collected works of Niels Bohr, covers the extension and refinement of the complementarity argument from 1933 until Bohr's death in 1962. All Bohr's publications on the subject, together with selected manuscripts and correspondence, are included. Part one contains Bohr's contributions to relativistic quantum theory, while the section part of the text, although also restricted in scope to the field of physics, address a broader audience. One of the most impressive articles is Bohr's own account of his debates with Albert Einstein, over more than twenty years, on the consistency, completeness and epistemological consequences of quantum mechanics.