This is a book of simple explanations. I have written it because the understanding of many of the topics with which solid-state physics is concerned is bedevilled by the fact that the physicist cannot see the physics for the mathematical hedge which besets him on all sides. Because of this, certain subjects are only open to the select few who have mastered some specialized skills. But however important a full mathematical treatment is for the detailed description of a phenomenon, there is often a simple physical model or argument which can be used to gain insight into a problem and it is this approach which has been used wherever possible. In this way I hope that the book will prove useful to the experimentalist who wishes to understand more about the processes which he proposes to investigate and also to the student who has learnt a little and wishes to explore farther.