III-V compound semiconductors are used in the fabrication of a variety of discrete and integrated optoelectronic devices due to their superior electronic properties. Diffusion is one of the fundamental processes employed in the semiconductor industry. This process plays a key role in the kinetics of many microstructural changes that occur during processing of semiconductors. Lattice defects robustly influence or even determine most of the important properties, for example the optical and electrical properties, of semiconductor materials. They may reduce the density of free carriers or mediate dopant diffusion and are thus of essential technological importance. Studying the formation of point defects responsible for the occurrence of diffusion and their behavior are of vital importance to the understanding of the properties of these materials and to their successful application in semiconductor devices. Positron annihilation spectroscopy is a powerful tool for detecting and studying the lattice defects in materials. This book is dedicated to investigations of crystal defects in III-V compound semiconductors by means of positron annihilation.