David Ford became leader of the party in 2001
The Alliance Party is the main cross-community party of Northern Ireland.
There are both Protestants and Catholics among its members and supporters.
It has sometimes been described as a "soft unionist" party because it believes Northern Ireland should stay in the United Kingdom until its people decide otherwise.
However, the party's principle aim is to urge Northern Ireland to rethink its entire mould of politics by leaving behind sectarian ties and community divisions in favour of seeing everyone as members of a single society.
The Alliance's sister party in Britain is the Liberal Democrats.
David Ford became leader in 2001 following the resignation of Sean Neeson.
In the 2001 election campaign, the party decided to withdraw candidates from five constituencies under a strategy to see as many pro-Good Friday Agreement candidates elected as possible.
Despite contesting 10 other seats, the party was not successful and has not had an MP elected since it was formed in 1970.
The Alliance Party took part in the political talks and supported the Good Friday Agreement, having campaigned for the deal in the 1998 referendum.
It, however, has since been critical of the way the Northern Ireland Assembly was set up, saying the rules can entrench sectarian positions and party tribalism.
Mr Ford has led his cross-community party during difficult times, as the political dynamic of bringing the DUP and Sinn Fein together has proved difficult terrain for the centre ground party.
The party retained its six assembly members in the 2003 election and has a number of local councillors.
In the 2007 assembly elections, Mr Ford was again elected in South Antrim. The party also gained a seat with the election of Anna Lo, the first person from an ethnic minority background to stand in the elections.
In 2009, the deputy leader of the party, Naomi Long, became the new lord mayor of Belfast.
She is the first woman to hold the post for nearly 30 years.
Mr Ford is likely to become the new Northern Ireland's Justice Minister.
In February, Northern Ireland's two main parties, the DUP and Sinn Fein, reached an agreement which, now the vote has been passed, will see a justice minister elected.
The Hillsborough Agreement allows for the first and deputy first ministers to identify a candidate who would command cross-community support in the assembly.
The Alliance Party's MLAs voted in support of the powers being devolved.
Mr Ford said it had been a momentous day for Northern Ireland.
"This is the start of the process which will see politics here come of age," he said.