Owen Smith joined the race to be Labour leader when the former shadow work and pensions secretary declared himself a "radical and credible" alternative who could take the party back into power.
Smith launched his bid the day after Jeremy Corbyn secured his place on the ballot paper for a contest expected to stretch through the summer, after the party's ruling National Executive Committee (NEC) ruled that only challengers, and not the incumbent leader, have to collect 51 nominations from MPs.
Angela Eagle is the other contender so far and here we take a look at the three candidates.
The embattled incumbent represents the hard left of the party and is the longest-serving of those standing to be leader. The 67-year-old was first elected in Islington North in London in 1983 and has held the safe seat ever since.
He supported Tony Benn against eventual winner Neil Kinnock in the 1988 Labour leadership battle and remained firmly on the party's back benches during its 13 years in power under Tony Blair and Gordon Brown. He was elected party leader in September after a surge in party membership, winning 251,417 votes, just shy of 60% of those cast.
He is a founder member and former chairman of the Stop the War Coalition, which was formed in 2001 and opposed the invasion of Iraq and war in Afghanistan. The grammar school-educated and Shropshire-raised politician was a trade union official and a Labour councillor in London before becoming an MP. A vegetarian, he has been married three times and has three sons.
The Wallasey MP was the first challenger to Corbyn to enter the fray this week and represents the soft left of the party. The 55-year-old Oxbridge graduate has represented the Merseyside seat since 1992, when she became its first ever Labour MP.
Born in Bridlington in Yorkshire, she went to school in Formby, Merseyside, before studying politics, philosophy and economics at St John's, Oxford. She later went on to work for a trade union. She held several minor posts during Labour's years in power, and spent five years on the Treasury Select Committee before becoming Exchequer Secretary to the Treasury and later pensions minister.
In opposition she was in Ed Miliband's shadow cabinet before Corbyn made her shadow first secretary of state and shadow secretary of state for business, innovation and skills when he was made leader. She resigned in June.
If she wins she would become the party's first gay leader. She is in a civil partnership with trade union activist Maria Exall. Eagle has an identical twin, Maria Eagle, the Labour MP for Liverpool Garston and Halewood and herself a former minister.
The least well-known of the candidates to throw their hat into the ring, Smith is also the newest MP and appears to sit politically somewhere between the other two. The 46-year-old has been MP for Pontypridd, the town in South Wales's Rhondda Valley where he was raised, since 2010.
Since his election he has held several shadow ministerial roles, including Treasury and health briefs. He is currently shadow work and pensions secretary opposite fellow Welsh MP Stephen Crabb.
Before becoming an MP he worked in several fields. He was a BBC journalist, including on Radio 4´s Today programme, before working in the biotechnology and pharmaceuticals industry. He also spent three years as special adviser to Welsh secretary Paul Murphy while Labour was in power. He is married with three children.