The behaviour of electrons in systems without periodicity is one of the most fascinating areas in solid-state physics, and the last 25 years have seen an enormous increase in research in this field. This has given rise to many new ideas for understanding electronic states in disordered systems, especially the study of the degenerate electron gas in which electron-electron interactions are important.This book provides a much needed survey of these advances. In the first part of the book, the authors discuss impurity bands in three dimensions. Attention is focused on the regime in which the electrons are spatially localized rather than free, so that an interesting interplay of localization and interaction arises. In the second part of the book, they look at the outstanding features of the two-dimensional systems, explaining how these make the localization problem special and interesting.The authors have provided a clear outline of the theoretical picture for the chosen materials and description heuristic. Each chapter is self-contained, allowing readers to pursue their special interests.