It is a peculiar experience to witness a small specialty in physics grow into an almost overwhelming field of science. Just 20 years ago, any interested physicist could easily read and process all the work on the magnetic properties of matter almost as a side activity. Today, around 500 papers on this subject are published each year, and various countries hold conferences and discussions on the magnetic properties of matter annually. The number of original papers presented at these events alone exceeds 200. The foundation for such development was laid by solid-state physics. The development and application of quantum theory, electron theory, and the theory of crystal defects have expanded our understanding of the relationships between the structure of matter and its magnetic properties.