This chapter discusses various types of point defects such as substitutional defects, interstitial defects, Frenkel defects, and color centers. It traces the discovery and development of metallic alloys and the rules governing their formation. Theoretical developments concerning color centers and alloy behaviour—such as phase rules, equilibrium phase diagrams, phase transitions, and ordering—are presented. The chapter also covers the historical developments of plastics and their deformation, types of dislocations, their movement and growth, grain boundaries, twinning, and the experimental techniques used to observe these phenomena.