Nuclear weapons were developed under the auspices of the United States' Manhattan Project during World War II. This book develops the underlying physics of the Manhattan Project at the level of an undergraduate physics student. Topics covered include basic concepts of nuclear reactions, discovery of the neutron, theory of fission, how the critical mass and efficiency of untampered and tampered fission cores are calculated, weapon yield, the electromagnetic and gaseous diffusion techniques of isotope enrichment, spontaneous fission and more. Appendices treat energy and momentum conservation in nuclear reactions, relevant physical and nuclear constants, a derivation of the neutron diffusion equation, and references. A number of analytic and numerical self-study questions with brief answers are provided.