This paper demonstrates a spatial approach towards the definition of localities for health care planning. Recent international decentralisation of health care provision, and more specifically devolution within the United Kingdom, emphasises the need to develop a geographical focus in the delimitation of local structures for health care planning. Geographers, but most especially those applying Geographical Information Science (GIS) techniques, have made enormous contributions in this field and more generally in research related to health services. This paper considers some of these previous approaches and moves on in the light of new technologies, and more importantly the availability of appropriate data, to create localities that reflect dynamic spaces of social interaction, administration and policy. The paper's focus is placed on the importance of flow data that reflects the spatial interaction between services and the population. This data, divided into three sub-groups of administration, education and health, allows us to identify the population's allegiance to place and ultimately create spatially bounded functional localities that reflect this. Whilst the approach is largely technology driven, it also incorporates the expertise of local health care professionals thus recognising the importance of collaboration and multi-sectoral engagement. Although this combined approach impacted upon the way in which the final localities were defined, crucially it enabled us to incorporate features of both rigorous spatial analysis and a wealth of local knowledge.