This volume is divided into five parts. The title of the volume refers primarily to part I, which is by far the largest and comprises papers discussing the fundamental questions of biology and related psychological and philosophical problems. Following the reproduction of papers brought to publication by Bohr, there is a separate Appendix to Part I including some of Bohr's most interesting and substantive unpublished contributions in this area. The papers in Part I span the last thirty years of Bohr's life and display his great interest in biological problems and his unremitting efforts to show that biology cannot be reduced to physics and chemistry. Part II contains articles of a more general cultural interest. Some of these show that Bohr regarded the complementary perspective to be of value also outside the scientific sphere. Part III contains the articles Bohr wrote about the great Danish philosopher Harald H(c)ffding. These short papers are presented in a section on their own because of the continuing discussion in the history of science about H(c)ffding's possible influence on Bohr's work in physics and his whole scientific approach. Part IV comprises articles illuminating the history of 20th century physics. Bohr had great veneration for his predecessors and teachers, and he prepared these articles with great care. Part V contains correspondence relating to the material in Parts I through IV. As in previous volumes an inventory of relevant unpublished manuscripts held at the Niels Bohr Archive constitutes an appendix to the whole volume.