The book comprises three parts. Part 1 gives a historical description of the development of ironworking techniques since the earliest times. Part 2 is the core of the book and deals with the metallurgical basis of microstructures, with four main themes: phase diagrams, solidification processes, diffusion, and solid state phase transformations. The rules for the use and interpretation of phase diagrams are described and illustrated with the aid of numerous updated iron-base diagrams chosen among the most useful for the understanding of steels. Solidification processes are discussed in two chapters, one on the fundamental concepts involved and the other on the analysis and interpretation of complex solidification structures. Special consideration is given to solid state phase transformations, including recent understanding of the bainite transformation. Part 3 begins by an introduction to steel design principles, describing the effects of the different alloying elements and the various strengthening mechanisms available. It then goes on to consider the different categories of steels, placing emphasis on their specific microstructural features. Finally, a comprehensive reference list includes several hundred pertinent articles and books, while a detailed index will be added to ensure ease of consultation. The book is the work of a single author, thus ensuring uniformity and concision. It is intended for scientists, metallurgical engineers and senior technicians in research and development laboratories, design offices and quality departments, as well as for teachers and students in universities, technical colleges and other higher education establishments.