Tight security because of the recent terror threat isn't the only reason that Brussels feels tense tonight.
Seeing khaki military trucks near the EU's main buildings, police on the streets locking down security before tomorrow's summit certainly changes the atmosphere. But for the prime minister, it's tense for a different reason.
By his own admission, David Cameron is trying to do something that has never really been tried before - change a country's relationship with the rest of the EU while already being a member.
What he is demanding are fundamental changes to the institution in a negotiation that will be mind-bogglingly complex. Trying something that is politically bold, even though the actual changes he wants to make are often criticised as being tame, comes with significant risk.
In the short-term, if he fails to persuade other countries that Britain should be able to limit benefit payments to EU workers in the UK he faces serious political embarrassment at home, not least from eurosceptics in his own party, including some cabinet ministers who have made this a red line.
The focus on that measure is intense - not surprisingly. It is a promise the PM made in the Conservative manifesto, an area of huge public concern and serious political conflict.
On the eve of the summit, there are all sorts of suggestions being made about the likelihood of a compromise. Several EU countries' officials have today made it absolutely plain that they have absolutely no intention of agreeing to the notion of Mr Cameron's four-year ban.
So there is feverish anticipation of what the alternatives might be. One EU source told me that Angela Merkel had told the PM to make three years, rather than four, his opening bid in the negotiations.
There's a suggestion that countries could agree to a proposal where during a so-called "ban", the same benefits would be paid, but the costs of those benefits could be covered by the worker's home country, rather than the UK.
It is though, a very fluid situation and there is no one proposal on the table that is likely to be agreed this week. Instead, the PM's goal is to inject a hefty dose of political momentum into the process so that officials can get on with the nitty gritty in the next couple of months.
In the next 36 hours, he needs his political counterparts to show they are willing to help.
But is there a risk that all the speculation over benefits distracts from the bigger issue?
In recent weeks there's been a building sense that the three other areas of reform are agreed. I understand that although there has been progress, concluding that those areas are somehow complete is a rather kind interpretation of where the discussions have really got to.
Talking this week to people familiar with the talks, there are nerves, a frustration even, that misleading assumptions are being made about the rest of the process.
In addressing the short-term political focus on whether a benefit ban could work, bigger questions about Britain's place in the EU as a country almost certain to never join the euro mustn't be ignored.