This book explores the modern physicist Niels Bohr philosophical thought, specifically his pivotal idea of complementarity, with a focus on the relation between the roles of what he metaphorically calls “spectators� and “actors.� It seeks to spell out the structural and historical complexity of the idea of complementarity in terms of different modes of the ‘spectator-actor’ relation, showing, in particular, that the reorganization of Bohr thought starting from his 1935 debate with Einstein and his collaborators is characterized by an extension of the dynamic conception of complementarity from non-physical contexts to the very field of quantum theory. Further, linked with this analysis, the book situates Bohr complementarity in contemporary philosophical context by examining its intersections with post-Heideggerian hermeneutics as well as Derridean deconstruction. Specifically, it points to both the close affinities and the differences between Bohr idea of the ‘actor-spectator’ relation and the hermeneutic notion of the relation between “belonging� and “distanciation.�