According to a first aspect of the present invention, there is provided a radiation source comprising: a reservoir configured to retain a volume of fuel; a nozzle, in fluid connection with the reservoir, and configured to direct a stream of fuel along a trajectory towards a plasma formation location; a laser configured to direct laser radiation at the stream at the plasma formation location to generate, in use, a radiation generating plasma; and a contamination filter assembly located in a fuel flow path of the radiation source, upstream of a nozzle outlet, a filter medium of that contamination filter assembly being held in place within the contamination filter assembly by a clamping force provided by one or more objects that at least partially surround the filter medium.