There is one area of the planning for London 2012 that is guaranteed to cause a sharp intake of breath among the key people at the Home Office, the Olympic Delivery Authority and Lord Coe's office: security.
It is the one subject that makes the mandarins jumpy.
There are many near certainties about London hosting the Olympics: it will definitely start on 27 July; the stadium will definitely be finished by then; there will have been endless heated arguments about budgets; and it will probably rain during the opening ceremony.
But security is one of the great unknowns. What kind of risk is there? How safe will the Games be? Will there be sufficient budget to eliminate terrorist threats? Can any sporting event of this magnitude possibly be 100% secure?
Assistant Commissioner Chris Allison was announced in the spring as the man at the Metropolitan Police Authority with overall control of security at London 2012.
In his first interview since taking up that position, he answered all my questions with a quiet and reassuring authority.
Allison has many years of first-hand experience of policing marches, demonstrations and football matches. He was also Gold Commander during the aftermath of the terrorist atrocities on 7 July 2005.
"The security strategy is already there," he told me for BBC Radio 5 live's Olympic special.
"We've been dealing with terrorists for 30 or 40 years in the UK.
"We are well used to delivering sporting events with a security overlay. What you will see is a subtle hand of security delivered by the police service to ensure a safe and secure Olympics."
Olympic budgets are the hottest of potatoes. Allison has £600m to fund his security operation, described recently by Home Office minister Lord West as the UK's "biggest security challenge since the Second World War". Is it enough?
"I am working within that envelope," he said. "If there were material changes to circumstances, then we also have a contingency of £238m that we may need to use. It's not going to be easy, but that what we are working towards.
"We need to be protectors of the public purse, but if there are new challenges then we need to discuss it.
"There are many assumptions that may change. If we consider we need more money, then we'll be in discussion with government.
"We are in financially challenging times, but equally we can't be reckless and take risks. Lots may change in the intervening two-and-a-half years."
If there's a massive overspend on frivolous hospitality or on poorly planned spectator or transport infrastructure, London taxpayers and those who hold the purse strings at Whitehall will not respond warmly. But if Allison goes over budget to make London 2012 secure, who will be bold enough to argue?
As he said: "This is the first Olympics Games to be staged against a 'severe' category threat. It is important that we put in place measures that make the Games safe to go to, but don't become so oppressive that people can't have a good time.
"We are aware of the methodologies of the various terrorist groups. We will have in place control measures that we hope will deter them.
"We have a great deal of experience in the UK in terms of structures and systems. As far as I'm concerned, it will be business as usual. We know what works and we will build on it.
"One hopes that we will have put in place measures that terrorists can't get through. We know what we have to do. We will deliver a safe Games. Terrorists change their methodology and we have to be smart in our tactics."
So what kind of Olympics can we expect?
"It will be a 'blue' Games using unarmed British constables policing it," he said. "We may call upon some niche assets from the military - such as fast boats on the waters of Weymouth."
Will he use armed officers on every corner? "There will be an element of that," he admitted, "but overwhelmingly you will see the unarmed British bobby."
So can London 2012 possibly be 100% safe and secure?
"There are always risks, from terrorism or protest groups or natural disasters," said Allison.
"We must mitigate against all those risks. In anything we do, there is always an element of risk. We need to minimise it and understand it."
You can hear Brian Alexander's exclusive interview with Assistant Commissioner Chris Allison tonight on BBC Radio 5 live Sport from 1900 GMT on Thursday 26 November.