This Field Guide covers the physics of semiconductors, from the materials used in optoelectronics and photonics to charge statistics and transport to pn junctions and their applications. It then addresses the physics of the interactions between radiation and matter at different levelsmacroscopic, microscopic, and quantum leveland includes the fundamental concepts of waveguides, fiber optics, and photonics devices such as light modulators. It finally highlights important applications of the field in engineering and applied physics. The guide summarizes the scientific and engineering foundations of optoelectronics and photonics and thus can be used as a textbook for college students, although it could be useful for practicing scientists and engineers as well.