This book has developed through a series of lectures on atomic theory given these last eight years at Chalmers University of Technology and several oth­ er research centers. These courses were intended to make the basic elements of atomic theory available to experimentalists working with the hyperfine structure and the optical properties of atoms and to provide some insight into recent developments in the theory. The original intention of this book has gradually extended to include a wide range of topics. We have tried to provide a complete description of atomic theory, bridging the gap between introductory books on quantum mechanics - such as the book