April 25th, 2003, marked the 50th anniversary of the publication, in Nature (171:737-738, 1953), of the paper by James D. Watson and Francis H. C. Crick describing the double-helical structure of DNA. While some would date the beginning of molecular biology to this paper, it is not difficult to find antecedent developments, experimental and conceptual, that point in the same direction. One such event that contributed to the origins of molecular biology is the lecture, “Light and Life,” delivered by Niels Bohr in Copenhagen on August 15, 1932. In this lecture, the text of which was later published in Nature (131:421-423, 457-459, 1933), Bohr attempted to apply his notion of complementarity, motivated by developments in atomic physics, to biology. Bohr’s ideas stimulated Max Delbrck, who attended the lecture, to switch from physics to biology. Once firmly engaged in the study of life, Delbrck proved to be a major intellectual force among the early pioneers studying bacteria and their viruses. Some curious aspects of these historical developments will be explored. In addition, the relevance of the notion of complementarity to modern biology and biomedical science will be considered.