Defects greatly affect the mechanical, electrical, optical, and magnetic properties of materials, especially semiconductors [1, 2]. There are many different types of defects, ranging from extended structures (e.g., grain boundaries, interfaces, dislocations, and precipitates), to complexes, to isolated native defects or impurities. The focus in this chapter is on localized defects such as vacancies, self-interstitials, isolated impurities, pairs and small complexes. These nanometer-size defects play many important roles and are the building blocks of larger defect structures. Understanding the properties of defects begins at this scale.