This multiple-case study examined how two national philanthropic foundations (Lumina Foundation and the Gates Foundation) conceptualized their postsecondary education priorities and the strategic actions they took to advance those priorities in the field between 2000 to 2023. Employing Fligstein & McAdam’s theory of fields framework and drawing data from documents, grant records, and interviews, I examined the how and the why of selected foundations’ postsecondary education priorities, strategic actions, and the decision-making processes that undergirded these choices. I also explored how the selected foundations operated in the postsecondary education field and how they positioned themselves in relation to other actors in the field as they sought to advance their specific set of priorities. Findings provide an in-depth analysis of how Lumina and Gates’ priorities and corresponding strategic actions shifted over time, going inside the black box of decision-making at the two organizations. For each of the two cases, study findings reveal three eras of grantmaking, three core postsecondary education priorities, and three corresponding strategic actions taken by each foundation. Over the study period, Lumina’s 1) student populations of focus shifted; 2) its outcomes of interest evolved from access to success and completion to the ‘equity imperative’; and 3) the foundation placed a greater emphasis on the diversification of its grantees. To advance these priorities in the field, Lumina helped build and sustain the postsecondary ecosystem – creating new intermediary organizations, providing technical assistance, building capacity, and developing coalitions and holding convenings. Lumina also shifted the expertise and perspectives of its staff. And, through its strategic priorities and actions to advance those priorities, Lumina helped shape the narrative surrounding postsecondary education. Over the same 23-year study period, Gates’ priorities focused on 1) data and data systems; 2) financial aid and finance; and 3) technology innovation and scale. Over the study period, Gates shifted how it invested its grant dollars to advance these priorities, increasing its investments in general operating support for its grantees. The foundation worked through intermediary organizations, building these organizations’ capacity, and creating opportunities for coalition building and convenings. And, Gates funded research to expand the postsecondary education evidence base. The findings of this study inform understanding of how philanthropic foundations use their discretionary resources for advancing lasting, systemic change in postsecondary education. Study findings also make theoretical contributions, expanding our understanding of how foundations operate in the field. Building on the study findings, this dissertation also offers several directions for future research.