Presbyterianism is a historically Reformed Protestant tradition named after its form of church government by representative assemblies of elders, known as "presbyters". Though other Reformed churches are structurally similar, the word Presbyterian is applied to churches that trace their roots to the Church of Scotland or to English Dissenter groups that were formed during the English Civil War, 1642 to 1651.

Presbyterian theology typically emphasises the sovereignty of God, the authority of the Scriptures, and the necessity of grace through faith in Christ. Scotland ensured Presbyterian church government in the 1707 Acts of Union, which created the Kingdom of Great Britain. In fact, most Presbyterians in England have a Scottish connection. The Presbyterian denomination was also taken to North America, Australia, and New Zealand, mostly by Scots and Scots-Irish immigrants. Scotland's Presbyterian denominations hold to the Reformed theology of John Calvin and his immediate successors, although there is a range of theological views within contemporary Presbyterianism. Local congregations of churches that use Presbyterian polity are governed by sessions made up of representatives of the congregation (elders), a conciliar approach as with other levels of decision-making (presbytery, synod, and general assembly). There are roughly 75 million Presbyterians in the world.

Presbyterianism's roots lie in the Magisterial Reformation of the 16th century. John Calvin's Republic of Geneva was particularly influential, along with Calvin's student, Scottish Reformer John Knox who worked with civil magistrates to establish the Presbyterian Church in Scotland, writing the book of common order and eventually the Scots Confession. Most Reformed churches that trace their history to Scotland are either presbyterian or congregationalist in government. In the 20th century, some Presbyterians played an important role in the ecumenical movement, including the World Council of Churches. Many Presbyterian denominations have found ways of working together with other Reformed denominations and Christians of other traditions, especially in the World Communion of Reformed Churches. Some Presbyterian churches have entered into unions with other churches, such as the Continental Reformed, Congregationalists, Lutherans, Anglicans, and Methodists. Presbyterians in the United States came largely from Scottish, Scots-Irish immigrants, and also from New England communities that were originally Congregational but changed because of an agreed-upon Plan of Union of 1801 for frontier areas.

Presbyterian tradition, particularly that of the Church of Scotland, traces its early roots to the Christian Church founded by Saint Columba, through the 6th century Hiberno-Scottish mission. Tracing their apostolic origin to Saint John, the Culdees practiced Christian monasticism, a key feature of Celtic Christianity in the region, with a presbyter exercising "authority within the institution, while the different monastic institutions were independent of one another." The Church in Scotland kept the Christian feast of Easter at a date different from the See of Rome and its monks used a unique style of tonsure. The Synod of Whitby in 664, however, ended these distinctions as it ruled "that Easter would be celebrated according to the Roman date, not the Celtic date." Although Roman influence came to dominate the Church in Scotland, certain Celtic influences remained in the Scottish Church, such as "the singing of metrical psalms, many of them set to old Celtic Christianity Scottish traditional and folk tunes", which later became a "distinctive part of Scottish Presbyterian worship".

Presbyterian history is part of the history of Christianity, but the beginning of Presbyterianism as a distinct movement occurred during the 16th century Protestant Reformation. As the Catholic Church resisted the Reformers, several different theological movements splintered from the Church and bore different denominations. The Presbyterian Church (The Church of Scotland) was a direct break off of the Roman Catholic Church as were the Anglican Church and the Lutheran Church.

Presbyterianism was especially influenced by the French theologian John Calvin, who is credited with the development of Reformed theology, and the work of John Knox, a Scottish Catholic Priest who studied with Calvin in Geneva and brought back Reformed teachings to Scotland. An important influence on the formation of Presbyterianism in Britain also came from John a Lasco, a Polish reformer, the founder of a Stranger's Church in London, based on the Geneva models.

The Presbyterian church traces its ancestry primarily to Scotland. In August 1560, the Parliament of Scotland adopted the Scots Confession as the creed of the Scottish Kingdom. In December 1560, the First Book of Discipline was published, outlining important doctrinal issues but also establishing regulations for church government, including the creation of ten ecclesiastical districts with appointed superintendents which later became known as presbyteries.

In time, the Scots Confession would be supplanted by the Westminster Confession of Faith, and the larger and shorter catechisms, which were formulated by the Westminster Assembly between 1643 and 1649.