Lecture notes from a series of lectures on nuclear physics given informally at Los Alamos Scientific Laboratory during the period of development of the atomic bomb. Presenters included: Felix Bloch, Robert F. Christy, C. L. Critchfield, Edwin M. McMillan, Emilio Segr,̈ Edward Teller, Victor F. Weisskopf, and John H. Williams. Topics included: terminology, radioactivity, neutron physics, two body problem, statistical theory of nuclear reactions, and diffusion theory. The notes were collected by M. E. Cieslicki during his employment at Los Alamos.