This classic of modern physics includes a vast array of approximation methods, mathematical tricks, and physical pictures that are also useful in the application of quantum mechanics to other fields. Students and professionals will find it an essential reference for calculations pertaining to hydrogen-like and helium-like atoms and their comparison with experimental results. In-depth explorations of the Dirac theory of the electron and of radiative effects include brief accounts of relevant experiments. The specific application of general field-theoretic results to atomic systems also receives a thorough examination. Author Hans A. Bethe (1906 2005), Professor of Physics at Cornell University, won the Nobel Prize in Physics in 1967. Co-author Edwin E. Salpeter is James Gilbert White Distinguished Professor of the Physical Sciences at Cornell University.