Apparatus and methods are used to control a polarization state of a radiation beam. A polarization control unit is configured to modulate a polarization state of at least a part of a radiation beam. A determination arrangement is configured to subsequently determine the polarization state of the at least a part of the radiation beam. A feedback unit is configured to provide signals to the polarization control arrangement based on at least the determined polarization state in order to correct for deviation in the polarization state of the part of the radiation beam from a desired polarization state. For example, the correction may ensure that the polarization state of the part of the radiation beam is at, or returns to, the desired polarization state.