A latent urbanization invades the rural world, making a landscape of compact settlements and dense population more fluid and dynamic. At the same time, urban diffusion resonates in rural districts, making their landscapes more heterogeneous and controversial, losing the ordering principles that underpin urban hierarchies and central locations. In this perspective, understanding urban complexity is a new frontier in regional studies and may contribute to a comprehensive analysis of the socioeconomic complexity that characterizes local systems in advanced economies. Moving away from the power relations between center and periphery, the socio-spatial structure of any city reinforces fragmentation, isolation, and fractal dimensions—the characteristic elements of contemporary metropolitan regions. At the same time, urban policies struggle to take shape due to the difficulty of identifying clear boundaries between cities and the surrounding rural area. The present contribution integrates relevant insights from regional science, town planning, rural development, empirical sociology, environmental assessment, and biophysical/ecological disciplines at large, shedding further light on the complex trajectories of local development in advanced economies, and the sustainability-competitiveness challenge in metropolitan regions.