This introductory chapter includes general remarks, definitions, and symbols and units related to atomic defects. Atomic or point defects are the simplest defects in a solid. Nevertheless a great number of physical and mechanical properties is sensitive to their presence. Extended one dimensional (e.g. dislocations), two dimensional (e.g. grain boundaries) and three dimensional (e.g. cavities) defects which are crucial to material behaviour. Knowledge of production mechanisms and properties of point defects is essential for assessing and understanding the atomistic as well as the macroscopic behaviour of materials.