This book is devoted to the study of the properties of materials that can be in a crystalline and glassy state. Its central focus is the physics of solids, whose structure is disordered, since the existing theories of solids are based on crystal structures. The approach adopted here is based on the comparison of data for crystals and glasses formed by the same atoms, paying particular attention to the under-explored glass-forming crystals. The book will be of interest to graduate students, solid states researchers, glass technologists, and young scientists beginning research in the field of experimental physics.