The category "NGO" (nongovernmental organization) is notoriously hard to fix. The term NGO-which defines these organizations in terms of what they are not-masks great diversity and assumes an unproblematic boundary. The use of the term persists, in no small part because several different types of actors involved in a range of fields depend on what we call the "productive instability" inherent to the NGO category. As a discipline, given our history and methodology, anthropology and anthropologists are uniquely poised to grapple with the ideas and practices associated with the inherently unstable category of NGO. Rather than attempting to fix the category or contest the boundaries implied by it, anthropologists are instead beginning to interrogate the meanings behind the contestations themselves.Rather than attempt precise classification or bemoan the uncertainty, we contend that the NGO category is "productively unstable." We argue that productive work lies ahead in charting similarities and differences within NGOs across aid and activism. This task mirrors an inherent messiness for both NGOs and anthropologists as we grapple with dilemmas of engagement. Such a critically engaged anthropology of NGOs also stands poised to offer useful guidance to the discipline as it struggles over "relevance" in this new century.