Properties of alloys are determined to a considerable extent by the form. the dispersity. the composition, and the quantitative relationships of the structural components. This monograph is an attempt to present the mechanism of the formation of the structure of alloys from the viewpoint of the most modern theory of phase transformations. The metastable state is treated at length be­ cause there are few data in the literature up to the present time. The book concerns the conditions determining the creation of the different phases in alloys. The forma­ tion of crystals of different compositions and dispersities is described. The cause of different degrees of metasta­ bility of alloys, the mechanism of the transformation of metastable systems into the stable state, and other problems are analyzed. The most widely used alloys were investigated. The authors tend to avoid demonstrations based on cumbersome calculations and whenever possible replace them with conceptual models. Some of the problems described here resulted from discussions during the seminar of the Metal Science Faculty of the Dnepropetrovsk Metallurgical Institute, directed by K. P. Bunin. The basic experiments were made by the author in the laboratory of the Department of Metal Physics of Dnepropetrovsk State University in collaboration with E. V. Finagin, A. N. Shul'diner, E. Z. Graifer, E.!. Psarev. !. I. Pesetskii, V. 1. L'nyanyi and I. S. Miroshnichenko.