National infrastructure is as much a priority in countries with a large stock of ageing infrastructural assets as it is for rapidly industrialising economies. Increasing attention is being paid to the risk and opportunities that the growing interdependence between infrastructural systems brings. However, there is no current methodology to enable long-term planning and future performance assessment of interdependent infrastructural systems. This paper makes the case for a long-term, cross-sectoral approach to analysing and planning the performance of national infrastructure systems. Such an approach is required to develop credible long-term visions for infrastructure provision and to analyse system performance across a range of possible futures. A preliminary version of a system-of-systems analysis for the UK is presented as an example. The implementation of a comprehensive, model-based approach is currently well under way.