This book is composed of ten chapters. Based on systematic description of research achievements of the project of ultra-fine grain steels and their engineering applications, new theories of microstructural refinement and the newly developed technologies in production of high strength and high toughness steels are introduced. The book features the integration of materials science with engineering technology. In the scope of theories of the strengthening and toughening of ultra-fine grain steels, the theory of deformation induced ferrite transformation (DIFT) for ferrite-pearlite steels has been put forward. The phenomenon of ultra-fine grain refinement effect by the existing precipitates of nanometer size in the steel produced by using thin slab casting and rolling (TSCR) has been discovered and analyzed. The theory of deformation induced precipitation and medium temperature phase transformation control for bainitic steels has been proposed. The theory of resistance against delayed fracturing of high strength and high toughness alloy structural steels has been established. In the aspect of production technologies, some production technologies for obtaining ultra-fine grains and high strength high toughness of steels are introduced. The chemical metallurgy, solidification technique, and welding technique etc. for ultra-fine grain steels are introduced. In the aspect of engineering applications of ultra-fine grain high strength and high toughness steels, all the trial applications and commercial applications in the areas of civil constructions, automobile manufacturing and engineering machinery etc. are described. The book provides theoretical concepts and engineering application technologies for the research, production, and application of ultra-fine grain steels.