In a governmental system, a party leader acts as the official representative of their political party, either to a legislature or to the electorate. Depending on the country, the individual colloquially referred to as the "leader" of a political party may officially be party chair, secretary, or the highest political office.
The party leader is often responsible for managing the party's relationship with the general public and leading the competition against political rivals, similar to the role of a party spokesperson. As such, they will take a leading role in developing and communicating party platforms to the electorate.
In many representative democracies, party leaders compete directly for high political office. It is thus typical in such states (notably in the Westminster system) for the party leader to seek election to the legislature and, if elected, to simultaneously serve as the party's parliamentary leader. In several countries utilizing the parliamentary system, if the party leader's political party emerges with a majority of seats in parliament after a general election, is the leading party in a coalition government, or (in some instances) is the largest party in a minority parliament, that party's leader often serves as the prime minister. In countries using the Westminster system, the leader of the largest political party not within the government serves as the leader of the opposition.
In Canada, the leaders of all major political parties are chosen during their respective political party's leadership conventions upon the completion of a leadership election. Exceptions to this process sometimes occur when Members of Parliament leave their former party to form a new party; examples of this include when Jean-François Fortin quit the Bloc Québécois to form Strength in Democracy in 2014 and when Maxime Bernier quit the Conservative Party to form the People's Party of Canada in 2018.
The leaders of communist parties often hold the title of general secretary (e.g. General Secretary of the Communist Party of the Soviet Union and General Secretary of the Communist Party of China) and the officeholder is usually considered the paramount leader of China. On 15 November 2012, Xi Jinping was elected General Secretary of the Communist Party of China at the 18th Communist Party national congress. 
The party organizations themselves and also their representatives (such as the chairperson, who is the party leader, and other board members) play a much more prominent role in German politics than they do in many other countries, where the parties are mainly represented by their members and leaders in government. Although the party leaders often also hold important public offices (such as government minister or parliamentary leader), those roles are clearly separated , even by law. Consequently, it does occasionally happen that the leaders of a German party are not even members of parliament, such as Saskia Esken and Norbert Walter-Borjans, who are currently the chairpersons of the (governing) Social Democrats. This sometimes leads to open conflicts between the party leadership, its parliamentary group and its members of government.
In the Netherlands, the party leaders are the most senior politicians within the political parties in the Netherlands. The leaders outwardly act as the 'figurehead' and the main representative of the party. Within the party, they must ensure political consensus. At election time the leader is always the Lijsttrekker (top candidate) of the party list. Outside election time the leaders most often serve as Parliamentary leader of their party in the House of Representatives, some party leaders opt to serve in the cabinet as a minister.
In the Third Rutte cabinet, the only party leader in the cabinet was Prime Minister Mark Rutte the leader of the People's Party for Freedom and Democracy (VVD), while the leaders of the other parties in the coalition: Christian Democratic Appeal (CDA) leader Sybrand van Haersma Buma, Democrats 66 (D66) leader Alexander Pechtold and Christian Union (CU) leader Gert-Jan Segers opted to remain as Parliamentary leaders in House of Representatives.
In the Republic of China, all of the political party leaders are elected by party chairmanship elections. The leader of the main ruling party Democratic Progressive Party (DPP) Cho Jung-tai was elected in the 2018 DPP chairmanship by-election on 6 January 2019. The leader of the main opposition party Kuomintang (KMT) Wu Den-yih was elected in the 2017 Kuomintang chairmanship election on 20 May 2017.
The method of selection of the party leader varies from party to party, though often it will involve an election involving all or part of the party membership. In some parties, only current members of the parliamentary party, or particular party office-holders, may vote; in others, such as the British Labour Party, though the entire membership is eligible to vote, some electors may have a much larger share of the vote than others (see also Superdelegate for a similar concept). If only one candidate emerges, they are said to be "elected by acclamation" or "ratified" by the general membership (sometimes the term "anointed" occurs informally or in media discourse).
The Prime Minister of the United Kingdom, Boris Johnson, gained the position on 24 July 2019; he has served as the party leader of the Conservative Party since 23 July 2019. Sir Keir Starmer is currently leader of the Labour Party and Leader of the Opposition.
If elected, political parties have party leaders in the executive branch of the United States government. The President, currently Joe Biden, becomes the de facto leader of their respective political party once elected, and the Vice President, currently Kamala Harris, likewise holds a leadership role as both the second-highest executive officer and the President of the Senate.
The legislative branch, otherwise known as the United States Congress, is made up of the upper chamber, the Senate, and the lower chamber, the House of Representatives, with party-elected leaders in each. The leader of the party with most of the representation (sometimes called the party-in-power) in each case is known as the majority leader, whereas the leader of the opposing party with the most members is known as the minority leader.
Party leaders in the United States Senate have been elected by their respective political parties' caucuses since 1913. They currently include President of the Senate Kamala Harris, President Pro Tempore of the United States Senate Patrick Leahy, Senate Majority Leader Chuck Schumer and Senate Majority Whip Dick Durbin on the Democratic side, and Senate Minority Leader Mitch McConnell and Senate Minority Whip John Thune on the Republican side.
The party leaders of the House of Representatives are elected by their respective parties in the House by secret ballot. The Republican Party is currently represented in the House by Minority Leader Kevin McCarthy and Minority Whip Steve Scalise, whereas the Democratic Party is represented by Majority Leader Steny Hoyer and Majority Whip Jim Clyburn. In the House of Representatives, the most powerful official is the House-elected Speaker, who is currently Nancy Pelosi of the Democratic Party.