World leaders, including US President Donald Trump, are all in the UK to have a meeting with each other.
Donald Trump met the Queen on Tuesday at Buckingham Palace and will join leaders from the other countries today, celebrating the 70th anniversary of Nato.
The meeting is being hosted by UK Prime Minister Boris Johnson, who will remind other leaders that as part of Nato they are friends and have a duty to defend each other.
But there are disagreements between world leaders at the moment, including President Trump and French President Emmanuel Macron who have fallen out over issues like taxes, trade and money.
Despite this, the two leaders sat down for a meeting on Tuesday.
All the 29 Nato leaders will try to put their differences to one side on Wednesday as they gather for a "family photograph" and then hold a three-hour meeting, covering issues such as cyber-attacks, Syria and China.
Nato stands for - the North Atlantic Treaty Organisation - it brings together the armies of various countries, including Britain and the United States.
It was formed in 1949, after the end of the Second World War.
The aim of Nato is to make sure that its 29 member countries don't fight each other; instead they use their forces to work together for world peace.
Nato has been used increasingly in situations where the world has decided urgent military action is needed.
This summit in December 2019 is special as it will be 70 years since the organisation was first set up and leaders will discuss how it can continue to keep people safe as well as different security challenges.
Trump's previous visits
In June 2019 during President Trump's state visit, there were protests as anti-Trump campaigners came out to express their unhappiness at his arrival in the UK.
It wasn't the first time people weren't pleased about him coming over here, back in 2018, campaigners flew a controversial balloon - which shows Donald Trump as a baby holding a mobile phone - in the skies over London.
When former prime minister Theresa May first invited Mr Trump for the state visit on behalf of the Queen back in 2017, more than 1.8 million people signed a petition calling for it not to go ahead.