Mixed-income development is the latest in a long line of policy interventions to improve outcomes in distressed urban neighbourhoods, particularly as an alternative to large scale public housing projects. Such developments are inherently integrationist, and have profound effects on the social lives and well-being of residents. After situating mixed-income developments within current residential demographic trends with regard to race and income, this article provides a comprehensive review of the 22 empirical studies addressing social integration in mixed-income developments in the United States, focusing on understanding the effects of spatial integration on social well-being. We discuss policy and practice implications to optimise the social well-being of residents, as well as strategies to improve scholarship in these settings.