This chapter is from the book Semiconductors and Semimetals: Imperfections in III/V Materials, which covers a range of topics from theory to materials issues, provides fundamental knowledge concerning imperfections in III/V compounds, and demonstrates the relevance of specific results for device performance and applications. The text examines microscopic models of structural and electronic defects in bulk and epitaxial III/V compounds. It provides an analysis of electronic properties in III/V compounds and discusses the influence of III/V compounds on device performance. The book should be of interest to students and researchers in materials science, electrical engineering, solid-state and device-oriented physics, surface science, and researchers, engineers and technicians in the semiconductor industry.