Complementarity between urban centres is a key element of Ireland‟s National Spatial Strategy and Northern Ireland‟s Regional Development Strategy. Both documents encourage the development of complementary roles for urban centres, at both the intraregional and inter-regional or national scale. In addition, at the intra-regional scale, different roles and functions are accorded to different types of urban components. Although the documents do not define complementarity, in most cases the term appears to signify functional distinctiveness/specialisation of urban centres. Such complementarity can be defined at different level of functional aggregation. Both documents are vague as to the level of aggregation and provide few clear suggestions as to the different roles or functions that are accorded to the various urban components.