This book is an introduction to the physics of highly excited, easily perturbed or interacting atoms. The book begins with a brief introduction to the traditional view of electron shells and their properties, and then goes on to discuss Rydberg states, quantum defect theory, atomic f-values, centrifugal barrier effects, autoionisation, inner shell and double excitation spectra, K-matrix theory, atoms in high laser fields, statistical methods, quantum chaos, and atomic effects in solids. The emphasis is throughout on radial properties, orbital collapse, many body effects, the breakdown of the...