Abstract: A method of and an apparatus for rapidly determining the polarization state of an incoming beam of light, such as from a fibre optic. The incoming beam is passed through a beam expander and then four separate portions of that beam are passed simultaneously through four Stokes filters respectively, arranged in a common plane. The four filtered beam portions are directed by respective lenses on to four detectors providing electrical signals indicative of the intensity of the respective filtered beams. From these four signals, the Stokes parameters may be deduced. The method allows very rapid assessments of the polarization state of an incoming beam to be achieved, and a bandwidth of several tens of KHz is attainable.