STEM learning is a process that unfolds through dynamic interactions over time and across settings. Formal education in schools is not the only--or necessarily the most significant--context for STEM learning. This paper outlines principles for building a diverse and connected ecosystem and the features of a STEM learning infrastructure to promote equity. The five design principles to support equitable learning across settings include the following: (1) Draw on Values and Practices to Articulate Shared Learning Goals; (2) Involve Stakeholders in Co-Design; (3) Make Connections across Settings; (4) Name Youth as Contributors; and (5) Intentionally Broker Learning across Settings. The authors' recommendations are derived from a review of literature on general strategies for leveraging diversity in STEM learning and on specific programmatic efforts to promote young people's learning across settings. The research on equity shares a premise that diverse everyday experiences are a resource for--rather than a barrier to--young people's learning. The goal of STEM education, then, should be not to eliminate perceived deficits in students, their families, or their communities, but to find connections between each of these and disciplinary knowledge and practices. Because the literature on programs that make explicit attempts to promote learning across settings is relatively new and sparse, the authors sought to identify programs that were grounded in this premise and that had some evidence of positive youth outcomes. The review included designs that show at least some promise of expanding youth access to STEM learning in and across settings. The result is a set of principles for designing equitable STEM learning ecosystems and a corresponding set of infrastructures necessary to support such systems.