The processing-microstructure-property relationships in steels continue to present challenges to researchers due to the complexity of phase transformation reactions and the wide spectrum of microstructures and properties achievable. This major two-volume work summarizes the current state of research on phase transformations in steels and its implications for the emergence of new steels with enhanced engineering properties. Volume 1 reviews fundamentals and diffusion-controlled phase transformations. After an overview of the historical development of phase transformations, chapters in part one discuss fundamental principles of thermodynamics, diffusion and kinetics as well as phase boundary interfaces. Chapters in part two go on to consider ferrite formation, proeutectoid ferrite and cementite transformations, pearlite formation and massive austenite-ferrite phase transformations. The third part discusses the mechanisms of bainite transformations, including carbide-containing and carbide-free bainite. The final section considers additional driving forces for transformation including nucleation and growth during austenite-to-ferrite phase transformations, dynamic strain-induced ferrite transformations (DIST) as well as the effects of magnetic fields and heating rates.