This chapter classifies and defines cities in South Africa from a migration vantage point. It first focuses on the differential urbanisation concept as a systemic framework that can explain how urban systems evolve and how the process determines the position and expected dynamism of individual cities within the urban hierarchy. Within this theoretical framework, the chapter reviews population redistribution patterns in South Africa since the mid- 1990s and compares them to population redistribution processes in the three largest urban agglomerations of South Africa. Finally, population redistribution movements in and around these core cities are used to explain: i) current morphological trends; ii) the relationship between the core cities and their surrounding cities; iii) the interpretation of functional and administrative space in terms of these trends. The chapter also reflects on the potential value to adapt the OECD methodology of functional urban areas to South Africa.