Given few knew who Jeremy Corbyn was before the summer, chances are that few will know who John McDonnell is either. Well, here's your quick guide.
He was first elected in 1997, having previously served on the Greater London Council as member for Hayes and Harlington (the constituency he represents in Parliament). He is chair of the Socialist Campaign Group of Labour MPs, and worked as a researcher and official for the National Union of Mineworkers and the TUC.
And here's what else you need to know about the new Shadow Chancellor:
1. John McDonnell is quite the Commons performer.
He famously grabbed the mace in the Chamber to express his outrage at the decision to approve a new runway at Heathrow - and was suspended from the Commons.
2. John McDonnell has some fruity rhetoric.
He said he would 'swim through vomit' to oppose the Welfare Reform Bill - so at least we now know where the party stands on this piece of legislation.
3. John McDonnell has the same attitude to party loyalty as Jeremy Corbyn.
Public Whip says he rebelled on 24.9 per cent of votes in the last session when Labour was in government. In Opposition between 2010 and April 2015, McDonnell still managed to rebel on 4.4 per cent of votes.
4. This is how John McDonnell describes Corbyn's economic policy:
So alongside deficit elimination, the Corbyn campaign is advocating a fundamental reform of our economic system. This will include the introduction of an effective regulatory regime for our banks and financial sector; a full-blown Glass-Steagall system to separate day-to-day and investment banking; legislation to replace short-term shareholder value with long-term sustainable economic and social responsibilities as the prime objective of companies; radical reform of the failed auditing regime; the extension of a wider range of forms of company and enterprise ownership and control including public, co-operative and stakeholder ownership; and the introduction of a financial transactions tax to fund the rebalancing of our economy towards production and manufacturing.
Public ownership does have an important role to play, but this will be through smart forms of 21st-century common ownership and control. For example, rail will be renationalised, but with a form of joint management involving workers and passenger representatives. Energy would be socialised from below by the massive expansion of renewable energy production and supply by local communities, local authorities and co-ops on the successful German model, removing the monopoly of the big six energy companies.
5. He praised the 'bravery' of the IRA.
In 2003, McDonnell told a gathering to commemorate IRA hunger striker Bobby Sands:
'It's about time we started honouring those people involved in the armed struggle.
'It was the bombs and bullets and sacrifice made by the likes of Bobby Sands that brought Britain to the negotiating table.'
He explained his comments here.
6. John McDonnell has stood for Labour leader...
...just like Corbyn, actually, but when the Hayes and Harlington MP stood in 2007 and 2010, he failed to gain sufficient nominations.