Although a completely satisfactory theory of nuclei has still to come into being, there does exist a widely accepted theoretical approach to the understanding of nuclear phenomena. Most workers in the field start from the same broad interpretations of the qualitative features of the data and most use the same, or closely related, methods of analysis. These basic ideas and techniques of nuclear theory constitute the principal subject matter of this book. However, some departures from generally accepted views may occur as a consequence of the need to select material for presentation, the selection being unavoidably influenced by the taste and judgment of the author.