The aim of this book is to introduce and explain important physical processes at the heart of the optical properties of semiconductor devices, such as light emitting diodes (LEDs) and semiconductor lasers. It is suitable for a half- semester (or a one-semester) course in Photonics or Optoelectronics at the graduate level in engineering physics, electrical engineering or material science. It offers an advanced analysis of the photo-physics of semiconductors, trying to avoid the use of exceedingly complex formalisms. Particular attention was devoted to offer a clear physical interpretation of all the obtained results. Various worked examples are added throughout all the chapters to illustrate the application of the various formulas discussed in the text.The book covers fundamental aspects of solid state physics, relevant for the calculation and analysis of semiconductor band-structure, and of quantum mechanics of electron-photon interaction. The photo-physics of bulk and quantum well semiconductors are discussed in detail. The final five chapters analyse the physics and properties of important photonic devices: light- emitting diodes (LEDs) and lasers, including Distributed Feedback (DFB) lasers, Vertical-Cavity Surface-Emitting Lasers (VCSELs) and Quantum Cascade Lasers. The general philosophy adopted in these chapters is the following: the fundamental physical processes are investigated, rather than the technological characteristics of the devices.