The Liberal Democrats, often shortened to Lib Dems, is a liberal political party in the United Kingdom formed in 1988 by the merger of the Liberal Party and the Social Democratic Party; the two parties had already been in an alliance for seven years prior to this, since not long after the formation of the SDP.

The Liberal Democrats is the third-largest party in the UK Parliament, behind Labour and the Conservatives, with 63 Members of Parliament (MPs) - 62 elected at the general election of 2005, they held Cheadle in the Cheadle by-election (July 2005) and gained one at the Dunfermline and West Fife by-election (February 2006). In the last session of the Scottish Parliament, the Scottish Liberal Democrats formed part of the coalition Scottish Executive with Labour; the Lib Dems supplying the Deputy First Minister, Nicol Stephen. The party's leader is Nick Clegg, who was elected on 18 December 2007.

Generally promoting social liberalism, the Liberal Democrats seek to minimise state intervention in personal affairs in the United Kingdom and throughout the world: many Lib Dem MPs criticise such intervention as symptomatic of a "nanny state". Unlike some other liberal parties, the Liberal Democrats were not founded on an explicit doctrine of economic liberalism, instead favouring combining a commitment to social justice and the welfare state with a belief in economic freedom and competitive markets wherever possible. The party's Presidential Book of Office, passed between outgoing and incoming Presidents, is John Stuart Mill's On Liberty.

The party is traditionally supportive of a multilateral foreign policy; they opposed British participation in the War in Iraq and support a swift withdrawal of troops from the country. The Liberal Democrats are considered the most pro-European party in British politics.

In the past decade the party has adopted a strong sense of environmentalist values - favouring taxing high polluters more than currently. Since the 2007 Liberal Democrat Party Conference in Brighton, the party favours cutting the basic rate of income tax by 'four pence in the pound', a significant alteration of their policy towards fiscal neutrality and away from increasing tax revenue for purposes of redistribution, that had previously been considered an "Old Labour" position.