Optical Nonlinearities and Instabilities in Semiconductors deals with various aspects of nonlinear optical phenomena and related optical instabilities in semiconductors. Measurements and explanations of the optical nonlinearities of various semiconductor materials and structures are presented, along with optical bistability and diode laser thresholds; self-oscillations; and chaos. This text consists of 17 chapters and begins with an introductory chapter to the historical background of investigations of the resonance-enhanced nonlinear optical properties of semiconductors and their manifestations in optical instabilities. The discussion then turns to the experimentally observed optical nonlinearities in homogeneous semiconductors and the microscopic theory of the optical band edge nonlinearities. This book considers the studies of the spectral region close to the band gap meant to exploit the resonance enhancement of the nonlinear optical behavior. The remaining chapters focus on nonlinear optical properties of semiconductor quantum wells; dense nonequilibrium excitations in gallium arsenide; optical decay and spatial relaxation; and optical bistability in semiconductor laser amplifiers. A chapter that describes instabilities in semiconductor lasers concludes the book. This book is intended for research students and active research workers who are interested in the basic physics or in the device applications of optical nonlinearities and instabilities in semiconductors.