Organizations with social missions, such as nonprofits and social enterprises, are under growing pressure to demonstrate their impacts on pressing societal problems such as global poverty. We draw on several cases to build a performance assessment framework premised on an organization’s operational mission, scale, and scope. We argue that, contrary to existing wisdom, not all organizations should measure long-term impact, defined as lasting changes in the lives of people and their societies. Rather, some organizations would be better off measuring shorter-term outputs or individual outcomes. Moreover, funders such as foundations and impact investors are best positioned to measure systemic impacts.