The most basic function of a beam splitter is to divide an incoming light beam into two or more beams with specific intensity ratios. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications. It operates based on the principles of reflection and refraction. Typically, a beam splitter is made of a transparent substrate, such as glass or fused silica, with a thin, precisely. Abstract Beam splitters form very important components of quantum photonic devices and this chapter presents a quantum description of the beam splitter. This division allows for the simultaneous analysis or utilization of the light's properties along two separate paths.