This monumental work focuses on the field of solid-state physics -- now more properly referred to as condensed-matter physics -- which grew to maturity between 1920 and 1960. The questions that were posed, and answered, during this period, regarding the structure of solid matter and why some materials are good insulators or conductors, led to applications of the highest economic, military and cultural importance. The history of these exciting developments is told in an easy-to-follow text, accessible to general readers, while maintaining standards of high scholarship.