Diffuse and low level light sources are best served with a photomultiplier for the detection of single photon emissions. Light detection and electron multiplication are statistical in nature and the mathematics of these processes is derived from first principles. The book covers other related topics such as scintillation counting, light guides, and large area detectors. The usually complicated subject of biasing a photomultiplier, very important for optimal performance, is reduced to a comprehensible set of calculations. All applications demand some form of electronics, the options for which are fully explored in this book.