Absent a formal definition or measure, it has not been possible to comprehensively identify rural-serving institutions (RSIs) for the purpose of research and policy making. The Alliance for Research on Regional Colleges (ARRC) spent a year working to resolve the absence of standards for identifying RSIs. The RSI Metric components are: (1) Percent of the institution's home county population classified as "rural"; (2) Average percent of adjacent counties' population classified as "rural"; (3) Population size of the institution's home county; (4) Whether the institution is in a county adjacent to a metropolitan area; and (5) Percent of the institution's awards conferred in Agriculture, Natural Resources, and Parks & Recreation. The authors found that RSIs enroll a considerable share of students from underrepresented backgrounds, and many are also minority-serving institutions. These institutions serve a larger share of students receiving Pell grants, as well as communities facing persistent poverty, low employment, and population loss. Given the important role they play, RSIs deserve greater investment from state legislatures, the federal government, and philanthropic gifts.