This study analyses periurbanisation as a specific process of changes caused by the expansion of cities towards rural areas that are situated at a greater distance from the core city but still within the framework of the functional urban area. Peri-urban areas are determined by a particular set of urban and rural characteristics that coincide, but seem to be dominated by the rural aspect. By using a trans-disciplinary approach based on multivariate analysis, the authors identified peri-urban areas within three Polish metropolitan areas. Results show that the areas mostly predestined to be peri-urban are territories classified as rural gminas (communes) and rural areas of urban rural gminas, which are peripherally located in relation to the main city. In general, the findings are relevant from the point of view of policy-making because they provide new insights into the very complex nature of peri-urban areas.