A patterning device for a photolithographic apparatus is used to form a patterned radiation beam, by imparting a cross-sectional pattern to the radiation beam during reflection from the patterning device. The patterning device comprises a layer of phase-change material that is capable of locally undergoing an induced structural phase change into respective ones of a plurality of stable and/or metastable states. Furthermore, the patterning device comprises a radiation reflective structure with periodically arranged layers adjacent to the layer of phase-change material. The radiation reflective structures do not partake in the phase changes. By locally changing the phase of the phase-change material, the reflectivity of the whole structure is modified, for example due to thickness changes in the layer of phase-change material that lead to destructive interference of different components of the reflected light or due to changes in surface roughness of the radiation reflective structure.