Constituency: Hackney North and Stoke Newington
Background: Diane Abbott was elected MP for Hackney North and Stoke Newington in 1987 and has served the constituency ever since. She was the first black woman to be elected to the House of Commons, and remained the only black female MP for ten years.
Before beginning her political career at Westminster City Council, she worked as a researcher in television. She now appears as a regular pundit on the BBC politics show This Week along with the former Conservative MP Michael Portillo, with whom she has been friends since schooldays.
Abbott is a notable campaigner on issues of race and education, and gave an award-winning speech in defence of civil liberties during the debate on the Counterterrorism Act 2008. She voted against the Iraq war, and is generally considered to stand to the left of New Labour.
She has one son from her marriage to the architect Richard Thompson (they divorced in 1993). The former Conservative MP Jonathan Aitken is her son's godfather. Aitken was her voting "pair" in the Commons for several years.
Notable supporters: One official nomination so far -- David Lammy.
Declared: 20 May 2010 on BBC Radio 4's Today programme.
Soundbite: "Labour needs the broadest possible contest. We can't go forward with a leadership debate where there is no woman."
Constituency: Morley and Outwood
Background: Ed Balls has been an MP since the 2005 general election. He was educated at Oxford and Harvard and worked for the Financial Times before his appointment as economic adviser to the then shadow chancellor, Gordon Brown, in 1994.
Balls has since worked as chief economic adviser to the Treasury and was promoted to minister for children, schools and families when Gordon Brown became prime minister in 2007. Widely seen as Brown's right-hand man, he was tipped for chancellor in the cabinet reshuffle of May 2009 before Brown chose to shore up the incumbent, Alistair Darling.
Among his policies implemented while in the cabinet are the scrapping of Sats for 14-year-olds and regulation of parents who home-school their children. He is married to Yvette Cooper, a fellow minister and MP for the neighbouring constituency of Normanton, Pontefract and Castleford. Together they were subject to allegations of "house-flipping" during the MPs' expenses scandal. They have three children.
Notable supporters: Officially nominated with 33 nominations, including Kevin Brennan and Vernan Coaker. Other supporters include Kerry McCarthy, Diana Johnson, Khalid Mahmood and Eric Joyce.
Declared: 19 May 2010 at a community centre in Gedling, Nottinghamshire.
Soundbite: "I think it's really important we don't just talk to ourselves. We've got to hear what the country's got to say."
Read more about Ed Balls's leadership campaign from Mehdi Hasan.
Campaign website: andy4leader.com
Background: Andy Burnham has served as the MP for Leigh since 2001. Born in Liverpool, he joined the Labour Party aged 14 during the miners' strike, before going on to study English at Cambridge. He worked in a number of roles for the Labour Party (including as a researcher for Tessa Jowell during the 1997 election) and is a member of the Transport and General Workers' Union. He has previously been associated with the Blairite wing of the party.
When Gordon Brown became prime minister in 2007, Burnham was elevated from junior ministerial ranks and held a number of cabinet roles, including chief secretary to the Treasury, secretary of state for culture, media and sport, and secretary of state for health.
He was exposed during the MPs' expenses scandal as having been engaged in several long-running disputes with the Fees Office over claims for a flat he was refurbishing. At one point, he wrote that "I might be in line for a divorce!" if he was not reimbursed within days for another claim. He is married with one son and two daughters, and is a keen cricket player and lifelong supporter of Everton FC.
Notable supporters: 17 official nominations so far, including Hazel Blears, David Blunkett and Gerry Sutcliffe.
Declared: 20 May 2010 at People's History Museum in Manchester.
Soundbite: "People from all backgrounds playing a part in reshaping the People's Party for a new century."
Read more about Andy Burnham's campaign from Mehdi Hasan.
Constituency: Doncaster North
Campaign website: edmiliband.org
Background: Ed Miliband was first elected as MP for Doncaster North in 2005. Born in London, he is the son of the late Marxist political scientist Ralph Miliband. He attended Haverstock Comprehensive School before reading PPE at Corpus Christi College, Oxford, going on to gain a Master's in economics at the LSE.
As a teenager, Miliband worked as an intern for Tony Benn, before joining the Labour Party as a researcher and speechwriter for Harriet Harman in 1993. He subsequently became an adviser to Gordon Brown before his election to the Commons in 2005. He served as minister for the Cabinet Office and chancellor of the Duchy of Lancaster from 2007, before being appointed secretary of state for energy and climate change in the cabinet reshuffle of October 2008.
He and his brother, David Miliband, are the first brothers to serve in the same cabinet since the Stanley brothers in 1938. He lives with his partner and their son in north London.
Notable supporters: Officially nominated with 45 nominations, including Hilary Benn, Frank Dobson, Sadiq Kahn, Emily Thornberry, Peter Hain and Chuka Umunna. Other supporters include Paul Murphy and Neil Kinnock.
Declared: 15 May 2010 in a keynote speech to the Fabian Society.
Soundbite: "I have empathy to unite Labour."
Constituency: South Shields
Campaign website: davidmiliband.net
Background: David Miliband was first elected as MP for South Shields in 2001. Born in London, he is the son of the late Marxist theoretician Ralph Miliband. He attended Haverstock Comprehensive School before reading PPE at Corpus Christi College, Oxford, before going on to get a Master's at the Massachusetts Institute of Technology.
After working in the voluntary sector and for the Institute for Public Policy Research, he was appointed Tony Blair's head of policy, and after Labour's victory in the 1997 general election became head of the prime minister's Policy Unit. Following his election to the Commons in 2001, he held a string of ministerial posts, including Cabinet Office minister and secretary of state for environment, food and rural affairs. After Gordon Brown became prime minister in 2007, he was made foreign secretary. He and his younger brother, Ed, were the first brothers to hold cabinet rank simultaneously since the Stanley brothers in 1938.
He is married to Louise Shackleton, a violinist in the London Symphony Orchestra, and they have adopted two sons from the US.
Notable supporters: Officially nominated with 54 nominations, including Alan Johnson, Douglas Alexander, Caroline Flint, Willie Bain and Tom Harris
Declared: 12 May 2010, outside the House of Commons on Labour's first full day out of office, flanked by 15 MPs who support him.
Soundbite: "New Labour isn't new any more. What I'm interested in is Next Labour."
Constituency: Hayes and Harlington
Background: John McDonnell was first elected as an MP in 1997. He left school at the age of 17 and held a string of unskilled jobs. He then studied for A-levels at night school before attending Brunel University. After gaining his Master's from Birkbeck he became a researcher and official with the National Union of Mineworkers and the Trades Union Congress.
He was elected to the Greater London Council in 1981. Following the abolition of the GLC, McDonnell was employed as head of the policy unit at Camden Council. He first fought his home-town seat of Hayes and Harlington in 1992, but lost by 53 votes. During the campaign, he was sued for libel by his Conservative opponent, Terry Dicks. The case was settled and the £55,000 damages and legal costs were funded through left-wing campaigning groups.
Since his election to the Commons, he has been a leading member of a number of all-party groups within parliament, including groups representing individual trade unions such as the RMT and the Fire Brigades Union. He rebelled against the government on several controversial votes, including the Iraq war, top-up fees and anti-terror laws.
Notable supporters: Five official nominations so far, including Frank Field and Jeremy Corbyn. Bob Crow, the RMT general secretary, Sunny Hundal, founder of Liberal Conspiracy, and Neil Clark have also declared their support.
Declared: 19 May 2010 in a speech to the Public and Commercial Services Union.
Soundbite: Leadership contest "stitched up from the start".
Read more about John McDonnell's campaign by Jon Bernstein.