Bohr, Delbrück and Schrödinger were physicists who had important influences on biology in the second half of the twentieth century. They thought that future studies of the gene might reveal new principles or paradoxes, analogous to the wave/particle paradox of light propagation, or even new physical laws. This stimulated several physicists to enter the field of biology. Delbrück founded the bacteriophage group which provided one of the roots of molecular biology. Another was X-ray crystallography which led to the discovery of DNA structure. The strength and success of molecular biology came from the many interactions between geneticists, physicists, chemists and biochemists. It was also characterized by a powerful combination of theoretical and experimental approaches.