Chancellor Philip Hammond is delivering his Budget in the House of Commons, promising investment to make Britain "fit for the future".
Announcements aimed at getting more houses built and some extra cash for the NHS are expected.
Mr Hammond, who has faced calls from some MPs to ease the government's austerity programme, will also update MPs on the state of the nation's finances.
Jeremy Corbyn will respond for Labour.
One area where there have been calls for an increase in spending is on public sector pay. BBC political editor Laura Kuenssberg said that while the chancellor would not announce extra cash for nurses' pay, he would promise the government would fund a rise if it is recommended by the independent pay body.
Before Mr Hammond got to his feet, Theresa May and Mr Corbyn clashed during Prime Minister's Questions.
Mr Corbyn called for more action against tax evasion, warning of Britain becoming a "tax haven". The PM said £160bn in tax had been collected because of the government's actions.
What did we already know would be in the Budget?
The chancellor is expected to announce more money for teacher training in England and extra cash to boost the numbers of students taking maths after the age of 16. And in a nod to younger voters, discounted rail cards will be extended.
An extra £2.3bn for research and development and £1.7bn for transport links are designed to address the UK's lagging productivity.
Extra money is also expected to be found for new charge points for electric cars and for the next generation of 5G mobile networks.
Expect the theme of innovation to ring through the speech, with Mr Hammond hailing the UK as being "at the forefront of a technological revolution".
Isn't the Budget normally in Spring?
Yes, that's the way it's been for the last twenty years. The last one was in March and normally there wouldn't be another one until Spring 2018.
But Mr Hammond thinks late autumn is a more suitable time for tax and spending changes to be announced and scrutinised before the start of the tax year in April. So from now on, Budgets will take place in November.
But aside from the timing, the choreography of Budget day remains the same.
While tradition dictates he can take a swig of his chosen tipple during his speech, Mr Hammond is expected to eschew anything too strong and confine himself to water during what is normally an hour-long statement.
Follow it on the BBC
- BBC Two and the BBC News Channel: Andrew Neil presents a Daily Politics Budget special from Westminster from 11:30 GMT with live coverage of Philip Hammond's statement and reaction from politicians and BBC experts. Jo Coburn will be reporting live from Peterborough.
- BBC Radio 5 live: From 12:30 GMT, Sarah Brett and Nihal Arthanayake will present a Budget special from Westminster and Manchester
- Online: Live updates and the latest analysis throughout the day
What's happened since the last Budget?
Quite a lot. In the last nine months, the UK has triggered Brexit and begun negotiations on the terms of its departure from the EU.
Economic conditions have changed too, although there is fierce debate about how much of this is attributable to uncertainty and negativity over Brexit.
Inflation has risen to 3%, its highest level in five years, while growth has faltered a little.
However, borrowing levels are at a 10-year low, giving Mr Hammond more flexibility, while employment remains at record levels.
The political backdrop has also changed enormously.
The loss of their majority in June's election sparked fresh Brexit infighting within the Conservatives.
The government has the backing of the DUP, but Mr Hammond – who is distrusted by many on the right of the party – does not have unlimited political capital in the bank.
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