Regional planning bodies throughout England are now in the early stages of preparing Regional Spatial Strategies (RSSs), which will succeed Regional Planning Guidance (RPG) as the frameworks guiding local development planning. This transition from RPG to RSSs is one outcome of the Planning and Compulsory Purchase Act 2004. Another outcome is the abolition of the Structure Planning function of English county councils, potentially resulting in a strategic vacuum in sub-regional planning. The Regional Assemblies (together with Regional Development Agencies and Government Offices) will need to address the subregional agenda, providing strategic leadership in what are often diverse and complex regional settings. The authors report on research undertaken on behalf of the South East England Regional Assembly and its partners. They examine both the changing policy context in this region, and the means by which a hitherto regional—and top-down—policy agenda is being ‘spatialised’ to address subregional diversity. In doing so, they also look at the move away from historic subregional boundaries, the redefining of ‘subregional’, and the shifting institutional ownership of the planning process at this particular tier.