With 44 days until the independence referendum the First Minister and the leader of the Better Together campaign are seeking to convince undecided voters.
Mr Salmond needs to close the gap in the polls showing a No vote still ahead and Mr Darling has to convince those yet to make up their mind that staying in the UK is the best for Scotland otherwise the balance could be tipped in favour of Yes.
Mr Darling is expected to challenge Mr Salmond on issues like the currency and membership of the European Union. He will argue that the SNP and Yes Scotland have not provided convincing enough answers that people should be persuaded to vote Yes.
At the same time the former Chancellor will argue the UK offers greater stability for Scottish jobs, pensions and benefits.
Blair McDougall, Better Together campaign director, said: "Voters in Scotland have been listening to the independence debate for over two years now. Surely if Alex Salmond had convincing answers on the details of separation, we would have heard them by now.
"Scots tuning in deserve to finally get straight answers from Mr Salmond to the questions they have been asking. What would separation mean for our pound, pensions and public services?
"Unlike the Leaders' Debates in the 2010 General Election, this debate isn't a job interview between candidates. It is a discussion about what separation would mean for our children and grandchildren's futures."
Mr Salmond is expected to argue that Scotland is a country whose natural resources and people make it wealthy enough to afford to protect public services better than the UK does.
He is expected to argue that the only threat to Scotland's membership of the EU is a Conservative government backing an in out referendum on Europe with Ukip enjoying support in England.
Yes Scotland Chief Executive, Blair Jenkins, said: "Independence is the opportunity of a lifetime for the people of Scotland, and the Yes campaign are looking forward immensely to the debate. We know that Scotland is one of the richest countries in the world, wealthier than the UK, France and Japan, and only the powers offered by a Yes vote will enable us to make this wealth work better for everyone in Scotland.
"Viewers will get the chance to hear why decisions made on Scotland's future should be taken here in Scotland. Our experience is that most undecided voters choose Yes when they hear both sides of the debate, and therefore we believe the mass TV audience will benefit our positive campaign."
The debate takes place tonight at the Royal Conservatoire, broadcast on STV. Both men will outline their case before being cross examined by one another then respond to questions from the audience.