This monograph explores the behavior of impurity atoms in semiconductors, integrating experimental data with their theoretical interpretation. The author begins by clearly explaining the basic physics of hydrogen-like impurities, atoms with partially filled electron shells, and amphoteric, isovalent, and gas-forming impurities to help readers understand the properties imparted by these impurities. The author also analyzes the macroscopic and microscopic mechanisms of the solubility and migration of impurities and defects in the crystal lattice. This book brings to light work done in the former Soviet Union and highlights several impurities that have potential, but have not yet found widespread application.