This book summarizes the present state of knowledge on the microscopic behavior of oxide surfaces. It includes classical approaches, introduces the concept of ionicity, and describes the mixed iono-covalent character of the oxygen cation bond in bulk materials. It emphasizes the characterisitics of the atomic structure (relaxation, rumpling and reconstruction effects), the electronic structure (band width, gap width, etc) and the excitations of clean surfaces. Intended for graduate students in material science, condensed matter physics and physical chemistry, this book proposes theoretical models to clearly illustrate the experimental results obtained on given systems. A comprehensive list of references is included.