Day one of the summit begins in the Belgian capital today (Picture: Benoit Doppagne/Belga/PA Images).
1. More British troops are heading to Afghanistan
Theresa May is to commit 440 additional British troops to the NATO mission in Afghanistan.
They will begin deploying in August with a second contingent to follow in February, taking the total UK military presence in the country to 1,100.
Around 16,000 troops from 39 alliance members and partner countries make up its Resolute Support Mission to train, advise and assist Afghan security forces and institutions.
2. More spending on defence?
Ahead of his arrival, Donald Trump reminded Europeans that the US had spent "many times more" on their defence than any other alliance member.
Mrs May was keen to emphasise the UK was one of just five alliance members to meet the target of spending 2% of GDP on its armed forces.
Countries including France and German have already agreed to up their defence spending.
3. What could President Trump do?
Following the break-up of the G7 summit in Quebec in June, Mr Trump's comments on defence spending are likely to increase nervousness among European and Canadian leaders about his commitment to the alliance and the wider international order.
If he decides his demands are not being listened to, the president could withdraw US troops from Europe.
There are currently around 65,000 American military personnel in Europe, down from several hundred thousand during the Cold War.
A withdrawal would weaken European defences against any Russian aggression.
4. Why does this matter now?
Donald Trump and Vladimir Putin are due to meet in Helsinki on 16 July following the US President's visit to the UK at the end of the week.
Any moves to weaken NATO, which was formed in 1949 as a bulwark against Russia, then called the USSR, would play right into Mr Putin's hands.