Jeremy Corbyn has come under fire for not saying he is sorry for anti-Semitism in the Labour Party in a BBC interview.
It's something that's been going on for a while now and Mr Corbyn has apologised to Jewish people in the past.
But when asked, four times, for an apology this week, Mr Corbyn didn't offer one.
He described racism as "a total poison" in our society and said he wanted to work with all communities to eliminate it.
Mr Corbyn has said this before, and in early November 2019 spoke about suspending members of the Labour party for anti-Semitic activity.
To understand the issue, you first have to know two very important definitions.
What's the difference between anti-Semitism and anti-Zionism?
Zionism is a political movement which supports the right of Jewish people to their own homeland in the land of their ancestors - modern-day Israel.
There are variations of Zionism - for example, some Zionists believe Israel has a right to some areas of land beyond its territory. Other Zionists disagree.
The vast majority of Jewish people are Zionists, though a small minority oppose Zionism, for either religious or political reasons. Non-Jewish people can be Zionists as well.
Anti-Zionism can be generally defined as opposition to the existence of the State of Israel. Anti-Semitism, on the other hand, means prejudice against Jewish people and has existed for centuries.
The State of Israel was declared in 1948. Since then, in some cases when people have strongly criticised Israel it has been difficult to know whether or not the criticism is motivated by anti-Semitism. This has led to accusations that anti-Zionism - rejection of the Jewish state - is merely a modern form of anti-Semitism.
Those who reject this comparison say this argument is used as a tool by Israel's supporters to silence reasonable criticism of Israel by portraying that criticism as racist.
What's anti-Semitism got to do with the left wing?
Anti-Semitism is not just a problem in the left wing of British politics.
In fact, historically, most Jewish people voted for the Labour Party, when it was supportive of the State of Israel.
But the party's stance on Israel began to change after the 1967 Middle East war - when Palestinians in East Jerusalem, the West Bank and the Gaza Strip came under Israeli occupation.
Traditionally the left wing has spoken up for the rights of oppressed minorities in the UK and around the world - and the Labour Party now aligns itself with the Palestinian cause.
Palestinians want their own state and face some severe restrictions from Israel. Israel says that, in the absence of any peace settlement, these measures are to protect it from Palestinian violence.
By the 1980s, people with strong views on Israel became more prominent within Labour. This contributed to the party developing an association with anti-Zionism, something which had previously been limited to the far-left.
What action has Labour taken?
This story dates back to 2016, when Mr Corbyn announced an independent inquiry into anti-Semitism and other forms of racism in the Labour Party.
The inquiry found that there is an "occasionally toxic atmosphere" of anti-Semitism within the party.
Speaking to the BBC in April 2018, the chairwoman of the inquiry accepted that her recommendations hadn't fully been implemented by the party.
They both gave anti-Semitism within the party as among their reasons for quitting - although it wasn't the only reason.
Then in February 2019, the MP Luciana Berger, who is Jewish, was the centre of a row within the party when her local branch tried to deselect her.
She has been critical of the party's handling of anti-Semitism and has also been the target of abuse online.
She was one of seven MPs to quit Labour at that time - although not all of them left due to concerns over anti-Semitism.
Then in May 2019, the Equality and Human Rights Commission launched a formal investigation into the Labour Party over allegations of anti-Semitism.
Labour said it would co-operate fully with the watchdog.
A party spokesman rejected "any suggestion that the party does not handle anti-Semitism complaints fairly and robustly, or that the party has acted unlawfully".
Where does Jeremy Corbyn come into all this?
The charge against the Labour leader isn't usually that he's anti-Semitic himself - but that he has been too slow to take action to stamp it out.
He has insisted time and time again there is no place for anti-Semitism in Labour.
The issue flared up again in March 2018, when Luciana Berger shared screenshots which showed a Facebook post from 2012.
The response from the Spokesperson is wholly inadequate. It fails to understand on any level the hurt and anguish felt about Antisemitism. I will be raising this further.— Luciana Berger (@lucianaberger) March 23, 2018
American graffiti artist Mear One had posted a picture of his street art, which was alleged to have been anti-Semitic and was eventually removed.
The allegation was that it promoted a conspiracy of powerful Jewish people controlling and oppressing others.
The six men all have exaggerated physical features often used to ridicule and stereotype Jewish people.
It emerged that Jeremy Corbyn had commented under the Facebook post, questioning the removal of the wall painting.
He has since expressed "sincere regret" for not looking more closely at the picture before he commented on it.
Labour's former Deputy Leader Tom Watson called it "a horrible anti-Semitic mural that was rightly taken down".
The artist Mear One denies it was an anti-Semitic piece of work, saying it was about "class and privilege".
Protests were held at the time against anti-Semitism and the Labour leader met Jewish leaders to address the issue.
The leaders claimed Mr Corbyn did not agree to any of the concrete actions they asked for, but he said he was "committed to rooting out anti-Semitism".
What do Jeremy Corbyn's defenders say?
Ever since he became Labour leader in 2015, Jeremy Corbyn has had to oversee a divided party.
Some of his allies say that the anti-Semitism row is a plot to undermine his leadership.
The seven MPs who quit in 2019 were all well-known critics of Corbyn and have said bullying within the party as well as its handling of Brexit were reasons they split.
Some people in the party deny the claim that he has not taken action to solve the problem.
They say that he has now speeded up the complaints system and is doing everything possible to rid the party of the problem.
In the 10 months up to January 2019, 96 members were immediately suspended from the party for their conduct and 12 were expelled.
Labour party figures also point to the fact that many complaints have been made about the Tories' attitude to Islamophobia.
There are calls from the Muslim Council to hold a full investigation into what is going on in the Conservative Party, just as the EHRC is undertaking a formal inquiry into the Labour Party.
A version of this article was first published in April 2018.