We present a survey of the state of the art in electron sources, concentrating on photocathode RF guns. These sources have played a key, enabling role in progress in a wide array of frontier scientific fields, ranging from driving very high gradient wakefield accelerators for high energy colliders, to providing the lasing medium from X-ray free-electron lasers (FELs). We discuss the physics of photoemission, and the role of advanced photocathode materials in lowering intrinsic emittance. We examine the critical roles of emission temperature and applied field on brightness limits. The description of cold, intense beams in such sources as single component plasmas is discussed, and this description is applied to the problem of space-charge induced emittance growth. Cutting edge new sources such as very high field cryogenic RF photoinjectors and ultra-high field “plasma photocathodes” are then introduced. Examples of the transformative applications in FELs, ultra-fast microscopes, and linear colliders are discussed.