非政府组织（NGO）是指那些独立于政府、不由国家创建的组织。这些组织通常是非营利性质的，至少部分资金来源于个人捐赠。虽然非政府组织在定义上包括以盈利为目的的企业，但该术语通常只用来描述那些非商业性、合法的、专注于社会文化或环境问题的倡导团体。

NGO与联合国或其指派的权威组织密切相关。2003年3月，绿色和平组织的成员曾爬上埃索/埃索美孚集团的建筑外墙进行抗议，事件中一些人质疑“NGO”一词的恰当性，因为它包含了所有非政府组织。因此，一些NGO更倾向于自称为民间志愿组织（PVO）。

据1995年的一份联合国报告统计，全球有近29,000个国际NGO。在国家级别，美国曾统计有超过两百万个NGO，其中大部分是在过去30年内成立的。在俄罗斯，有65,000个NGO。每天都有数十个新的NGO成立，例如，肯尼亚每年有240个NGO成立。

NGO的历史虽然可以追溯到志愿者协会的早期存在，但今天我们所熟知的处于一线的大型NGO是在过去两百年间发展起来的。最早的一个之一是1863年成立的国际红十字会。 term "non-governmental organization" emerged in 1945 with the establishment of the United Nations, mentioned in Article 71, Chapter X of the UN Charter, as an advisory role for NGOs without government or state membership (see Consultative Status). The definition of "international NGOs" (INGOs) was first proposed by ECOSOC resolution 288(x) on February 27, 1950, as "any international organization which is not established by an international treaty."

The role of NGOs and other major groups in sustainable development was recognized in Agenda 21, Chapter 27[2], aiming to rectify the relationship between the UN and NGOs. The process of globalization in the 20th century increased the importance of NGOs, as many issues could not be resolved within national borders. International treaties and organizations like the World Trade Organization were considered too focused on the interests of capitalist enterprises. To balance this, NGOs focused on humanitarian aid, development funding, and sustainable development. The World Social Forum is a notable example, offering a counterpoint to the World Economic Forum held annually in Davos, Switzerland. The fifth World Social Forum was held in Porto Alegre, Brazil, in January 2005, with representatives from 1,000 NGOs.

NGOs can be categorized by various criteria. The World Bank uses the following classification method:

- Operational NGOs: Their primary goal is to design and implement projects related to development. They can be further categorized as "relief-oriented" or "development-oriented," based on their service focus. Additional classifications can be made based on whether they have religious affiliations, their long-term or short-term focus, and whether they primarily serve the public or private sectors. Operational NGOs can be based on local, national, or international groups.

- Advocacy NGOs (有时被称为好战型NGO): Their main purpose is to defend and promote a particular cause. In contrast to operational NGOs, these organizations typically aim to raise awareness through lobbying, publications, and the activities of militants.