During the period 1949–1961 Szigeti published four seminal papers on the dielectric behaviour of crystals. Szigeti's theory is applicable to isotropic and anisotropic, ionic and covalent crystals with different structures. Szigeti's theory connects dielectric, spectroscopic and elastic properties. An important outcome of Szigeti's theory is the concept of the effective ionic charge (𝑠). It is pointed out that 𝑠 correlates with a number of physical properties and is a measure of ionicity of the interatomic bond. Since Szigeti's work, several theoretical models have been proposed to account for the fact that $s < 1$. These models provide an insight into the complex polarization mechanisms in solids. This review summarizes Szigeti's work and the work that followed; the implications and applications of Szigeti's theory are discussed. Some new results are also included.