Many of the imaging, sensing, and information processing applications discussed in this book are based on optical systems. In addition, even in atomic, ionic, and solid-state systems, electromagnetic waves are used to excite the systems to higher energy levels, to manipulate superpositions of states, to move atoms around, and to measure the states via fluorescent readouts. As a result, optical sources and detectors are vital ingredients in applications, regardless of the primary physical platform being used. In this chapter, we look at some common optical sources and optical detectors and discuss some of their basic properties.