Beam splitters form very important components of quantum photonic devices and this chapter presents a quantum description of the beam splitter. Output states from beam splitters under different inputs such as single photons entering through one port, two photons entering through the two input ports, single photon in a multimode state, and entangled photons are discussed. Optical directional couplers as beam splitters are also presented. Mach-Zehnder interferometer which uses two beam splitters is presented showing single photon interference. Interaction-free measurement is also presented.