In the past two decades our understanding of the occupied electronic states of solides has undergone a revolution, while our knowledge of the unoccupied states has lagged behind. This is now changing, owing to the progress in techniques such as X-ray absorption and inverse photoemission, and a complete picture is beginning to emerge. This book presents the theoretical and experimental basis of the subject of unoccupied electronic states. It begins by describing the modern theoretical picture of unoccupied states, starting with the single-particle picture and going on to various aspects of many-body interaction and correlation. The theory of modern spectroscopic methods (XANES, EELS,IPS and BIS) used to study unoccupied states is discussed and examples are given to illustrate these techniques. This volume stresses the unity of the concepts required to understand both occupied and unoccupied states and demonstrates the importance of unoccupied states for our comprehension of the optical, thermal and transport properties of materials.