The concept of a structure element is crucial in studying the thermodynamics of non‐stoichiometry in solids. This chapter discusses the structure elements of a solid; quasi‐chemical reactions in solids; equilibrium states between structure elements in solids; and thermodynamics of structure elements in unary solids. A solid is said to be binary if it contains at least two structure elements occupied by atoms or ions. We sometimes encounter two different chemical elements on two different types of sites. It pertains, for example, to metal oxides, sulfides, halogenides, etc. In numerous solids there are differences, sometimes very slight, in the ratio between the quantities of the two elements, but the ratio of the quantities of sites is still the same as in the ideal solid. These solids are said to be non‐stoichiometric. All quasi‐chemical reactions involving structure elements, and in particular, point defects obviously involve equilibrium constants that need to be determined.