Spatial planners and policy makers currently struggle to understand the peri-urban area, with its mixture of land uses and its transitional status between the urban and the rural. This paper presents the concept of transition, derived from complexity science, to allow planners to analyse peri-urban development in terms of a number of interacting processes, some induced, some evolving autonomously. Drawing on four case studies of European urban regions, the research finds that many of the dynamic processes underlying peri-urban development are not susceptible to the influence of planning agencies. This should enable planners to develop a more adaptive approach in the future, identifying areas where productive and case-specific interventions ...