The periodicity of the lattice in a defect-free crystal determines the mass and charge density. However, the introduction of a point defect disrupts this regular pattern. In metals, the presence of conduction electrons enables effective electronic screening of defects, resulting in uncharged point defects. Conversely, in ionic crystals, the formation of a point defect, such as a vacancy in one sublattice, disturbs the overall charge neutrality. To maintain charge balance, defect populations in ionic crystals can exhibit Frenkel disorder or Schottky disorder. Frenkel disorder involves an equal number of vacancies and self-interstitials in one sublattice, while Schottky disorder entails an equal number of vacancies in both, the cationand anion-sublattices. For instance, in compounds like NaCl, which consists of cations and anions with opposite charges of equal magnitude, the number of vacancies in each sublattice must be equal to ensure charge neutrality. In semiconductors, point defects introduce energy levels within the band gap, and their charge state (neutral or ionized) depends on the position of the Fermi level. In the subsequent sections, we will first examine point defects in metals, and ionic crystals before going on to semiconductors.