Income mixing is strongly endorsed as a principle in housing and community development because it is expected to provide social diversity, help low-income people get access to higher-quality goods and services, and achieve social and economic integration. Yet mixing may also pose challenges, and homogeneity may have benefits that should not be abandoned. This paper suggests that the potential benefits of income mixing can be maximized by attending to geographic scale more carefully than has occurred in the past. It reviews policies across multiple scales and proposes research to understand how income mixing works at various scales.