Solids, in general, comprise both metallic and nonmetallic, organic and inorganic, crystalline as well as noncrystalline materials. In this work we shall restrict ourselves to a discussion of the properties of inorganic, nonmetallic crystals, paying attention in particular to the role played by imperfections. The substances to be discussed vary widely and include elements like Ge, Si C, Se, and Te, simple compounds like the alkali halides (e.g. NaCl), silver and copper halides (AgBr and Cui), III-V compounds (InSb, GaAs, and others), II-VI compounds (BaO, ZnS, CdTe, and others), SiC, more intricate two-component compounds like Cu2O, SbsS3. FeSs. H2O, Bi2Te3. And Fe3O4. And ternary compounds like the ferrites (e.g. MgFe2O4) and chalcopyrites (CuFeSx). The study of such crystals involves chemistry and physics, with engineering (electronics) a highly useful aid in carrying out experiments. The present discussion will be concerned mainly with the chemical approach to the problems.