Iron played an important role in the development of the industrial society and has not lost any of its significance since today. This book provides the foundations of understanding the physical nature of iron and its alloys. Basics and recent developments concerning its constitution and magnetism are presented as well as its thermal properties. The exceptional role of iron with its wide spectrum of most different technological and physical properties relies on its versatility, its polymorphism of its crystal structure and its magnetism. Therefore it is the aim of the book to link together the constitution and magnetism of iron.