This editorial introduces the concept of the internal border and its individual contributions to a Special Issue on the topic. It discusses the shift from external borders to diverse sites and actors within the territory of the state, emphasizing the city and urban space as crucial sites for understanding the internal border. The internal border is defined as a mechanism of ordering and othering that creates divisions between insiders and outsiders, citizens and non-citizens, along with racialization, ethnicity, gender, class, and health. The introduction explores the internal border's role in shaping social differences and its connection to external borders. It highlights the need to study the border through the city, embedded within a multiscalar and relational framework. The Special Issue includes articles that analyze the internal border in various cities and urban spaces, focusing on multiple actors, sites, and legal categories of migrants. The contributions reveal the complex dynamics of inclusion and exclusion, the interplay between internal and external border controls, and the role of civil society organizations in shaping the border. The issue aims to contribute to the understanding of the internal border as a mechanism of socio-spatial differentiation and to decentre the state-centric view of borders.This editorial introduces the concept of the internal border and its individual contributions to a Special Issue on the topic. It discusses the shift from external borders to diverse sites and actors within the territory of the state, emphasizing the city and urban space as crucial sites for understanding the internal border. The internal border is defined as a mechanism of ordering and othering that creates divisions between insiders and outsiders, citizens and non-citizens, along with racialization, ethnicity, gender, class, and health. The introduction explores the internal border's role in shaping social differences and its connection to external borders. It highlights the need to study the border through the city, embedded within a multiscalar and relational framework. The Special Issue includes articles that analyze the internal border in various cities and urban spaces, focusing on multiple actors, sites, and legal categories of migrants. The contributions reveal the complex dynamics of inclusion and exclusion, the interplay between internal and external border controls, and the role of civil society organizations in shaping the border. The issue aims to contribute to the understanding of the internal border as a mechanism of socio-spatial differentiation and to decentre the state-centric view of borders.