More than half the urban population in Africa, Asia, and Latin America lives in urban centres with less than half a million inhabitants, many of them in market towns and administrative centers with between 5,000 and 100,000 inhabitants. However, the demographic significance is not the only reason for an interest in these centers. The economic interdependence between urban-based enterprises and rural consumers and between rural producers and urban markets, and the reliance of many households on both rural and urban-based resources are often stronger in and around small and intermediate urban centers, underlining their important potential role in local economic development. And for much of the world’s population, the services needed to achieve many of the Millennium Development Goals are located in small and intermediate urban centers, to serve their populations and those living in surrounding rural areas.

Although most low- and middle-income nations have experienced rapid urbanization over the last few decades, there are large variations between nations in the scale and spatial distribution of urban change, including the roles and functions of small and intermediate urban centers within their national urban systems. This makes generalizations problematic and unhelpful in policy formulation and points to the need to understand such variations and the local factors underlying urban change within different nations. The significant differences in how national governments define urban centers also limit the validity of international comparisons of urban growth trends.