Quakers are people who belong to a historically Protestant Christian set of denominations known as the Religious Society of Friends. Members of these movements ("the Friends") are generally united by a belief in each human's ability to experience the light within or see "that of God in every one". Some profess a priesthood of all believers inspired by the First Epistle of Peter. They include those with evangelical, holiness, liberal, and traditional Quaker understandings of Christianity. There are also Nontheist Quakers, whose spiritual practice does not rely on the existence of God. To differing extents, the Friends avoid creeds and hierarchical structures. In 2017, there were an estimated 377,557 adult Quakers, 49% of them in Africa.

Some 89% of Quakers worldwide belong to evangelical and programmed branches that hold services with singing and a prepared Bible message coordinated by a pastor. Some 11% practice waiting worship or unprogrammed worship (commonly Meeting for Worship), where the unplanned order of service is mainly silent and may include unprepared vocal ministry from those present. Some meetings of both types have Recorded Ministers present; Friends recognised for their gift of vocal ministry.

The proto-evangelical Christian movement dubbed Quakerism arose in mid-17th-century England from the Legatine-Arians and other dissenting Protestant groups breaking with the established Church of England. The Quakers, especially the Valiant Sixty, sought to convert others by travelling through Britain and overseas preaching the Gospel. Some early Quaker ministers were women. They based their message on a belief that "Christ has come to teach his people himself," stressing direct relations with God through Jesus Christ and direct belief in the universal priesthood of all believers. This personal religious experience of Christ was acquired by direct experience and by reading and studying the Bible. Quakers focused their private lives on behaviour and speech reflecting emotional purity and the light of God, with a goal of Christian perfection.

Past Quakers were known to use thee as an ordinary pronoun, refuse to participate in war, wear plain dress, refuse to swear oaths, oppose slavery, and practise teetotalism. Some Quakers founded banks and financial institutions, including Barclays, Lloyds, and Friends Provident; manufacturers including the footwear firm of C. & J. Clark and the big three British confectionery makers Cadbury, Rowntree and Fry; and philanthropic efforts, including abolition of slavery, prison reform, and social justice. In 1947, in recognition of their dedication to peace and the common good, Quakers represented by the British Friends Service Council and the American Friends Service Committee were awarded the Nobel Peace Prize.