The prime objective of the collection of lectures contained in this book is to formulate our current understanding of aperiodic solids with long range order, quasicrystals in particular. At the same time it provides a comprehensive insight into the most recent concepts and tools of condensed matter physics and gives a detailed discussion and interpretation of relevant experimental data. The deliberate pedagogical approach of the authors makes this book particularly useful for graduate students and experimentalists wishing to acquaint themselves with the fundamental concepts.