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 independent particle approach, the emergence of chaos, and the behaviour of atoms inside clusters and solids. A very full account of autoionisation includes not only the standard treatment for isolated resonances, but also several alternative approaches. The book discusses many experimental examples and has many diagrams and a comprehensive reference list.