Non-governmental organizations (NGOs) are legally constituted private organizations that are not part of any government. They range from small grass-roots organizations to large trans-national organizations. Roles vary across NGOs, and include advocacy, governance, and service delivery. NGOs can operate independently, or within existing health structures, such as primary health care, or in public–private partnerships. They play an important role in public health by bringing services to people who might not otherwise have access to these, and have shown to improve health, and reduce health inequities and disparities. NGOs' advocacy and governance has resulted in them being powerful influencers in both national and international agenda setting, the formation of intergovernmental agreements, and national and international trade agreements. Criticisms include effectiveness, accountability, autonomy, and neoliberal arguments of rolling back the state.