Publisher Summary This chapter discusses the properties of vacancies and self interstitials, their interaction with other lattice defects, the production mechanisms, and their importance for radiation damage of materials. Point defects are the lattice defects of zero dimensionality—that is, they do not possess lattice structure in any dimension. Typical point defects— vacancies and self-interstitials—are impurity atoms in a pure metal. Vacancies are produced simply by heating a crystal, in concentrations sufficiently high for quantitative investigations. To obtain similar concentrations of interstitials, they must be produced by doing external work on the crystal. Such work is done on an atomic scale by energetic particle irradiation. Collisions between the projectiles and lattice atoms cause displacements of atoms from substitutional sites to interstitial sites. Thus, vacancies and interstitials are produced in equal numbers. Plastic deformation also produces vacancies and interstitials. Although its application is much less expensive than particle irradiation, it has not become a common procedure for point defect production because it does not allow a controlled defect production independent of complex dislocation networks. Anomalously high point defect concentrations occur in some non-stoichiometric intermetallic compounds. Here, vacancies and interstitials clearly play the role of additional alloying elements and are of thermodynamic significance.