This book presents an in-depth discussion of the semiconductor-laser gain medium. The optical and electronic properties of semiconductors, particularly semiconductor quantum-well systems, are analyzed in detail, covering a wide variety of near-infrared systems with or without strain, as well as wide-gap materials such as the group-III nitride compounds or the II-VI materials. The important bandstructure modifications and Coulomb interaction effects are discussed, including the solution of the longstanding semiconductor laser lineshape problem. Quantitative comparisons between measured and predicted gain/absorption and refractive index spectra for a wide variety of semiconductor-laser materials enable the theoretical results to be used directly in the engineering of advanced laser and amplifier structures. A walth of examples for many different material combinations bestow the book with quantitative and predictive value for a wide variety of applications.