We focus on the notion of borders to explore how mobility and immobility in the city affect the relationship between human development and urban culture. We define borders as a relational space made of territoriality, representations and different possibilities of mobility and immobility. Drawing on research in the favelas of Rio de Janeiro, we suggest a systematic approach to the analysis of borders and identify the socio-institutional, spatial and symbolic elements that make them more or less porous and thus more or less amenable to human mobility. We highlight the association between porosity in city borders and human development and illustrate the model contrasting two favela communities in Rio de Janeiro. We show that participation in the socio-cultural environment by favela grassroots organisations increases the porosity of internal city borders and contributes to the development of self, communities and the city. To focus on borders, their different elements and levels of porosity means to address simultaneously the psychosocial and cultural layers of urban spaces and the novel ways through which grassroots social actors develop themselves through participation and semiotic reconstruction of the socio-cultural environment.