General formulation of Thermodynamics of Metastable States and Relaxation is proposed and discussed. Thermodynamics is used also as a tool for solving problems in Materials Science. Wide range of materials are considered. Among them crystals, containing micro and macro defects, like point defects, vacancies, dislocations, pores and cracks. Influence of these defects on properties of materials, including strength and fracture ones, is discussed. Polar materials, containing pores and inclusions of different phases are investigated. Cavitation Phenomemon in polar materials in external fields is predicted. Semiclassical theory of the basic properties of nanoobjects is formulated. Friction of collective electrons in nanoobjects is taken into account. Comparison of experimental data with theoretical ones show rather a good agreement. This book can be useful for Master Degree and Ph.D. Students, studying Materials Science.