In this chapter, we present the origins and definition of inner peripheriesInner peripheries. Our discussion begins by highlighting contemporary trends in the transformation of the EU Cohesion PolicyEU Cohesion Policy, which involve the reorientation of its objectives, a shift in the understanding of proximityProximity, and the increasing importance of the subregional level for identifying and addressing spatial disparities in socio-economic development. These factorsFactors are seen as particularly relevant to the concept of inner peripheriesPeripheries. We refine our understanding of inner peripheries by elaborating on the theoretical foundations of the concept, which are based on a triangulated theoretical-methodological-narrative approach. This approach integrates the concept of geographical peripheralityPeripherality, the Modern World System theory, the foundational economyFoundational economy framework, and theories of spatial justiceSpatial justice. Additionally, we justify the significance of Functional Urban Areas (FUAs) in the delimitationDelimitation of inner peripheries, which serve as the foundation for the spatial framework adopted in our research. Finally, we propose an original analytical framework for inner peripheries, drawing on the concept of territorial capitalTerritorial capital and a systematic analysis of development factors.