8 June 2013 Last updated at 14:05 GMT Help
Tens of thousands of people are expected at a rally in London's Hyde Park to look at the problem of world hunger.
Speakers include Microsoft founder Bill Gates, who set up "If..." with Archbishop Desmond Tutu, and former Archbishop of Canterbury Rowan Williams.
Mr Gates told the BBC: "We need to be more generous and we need to pay attention to what's going on in these poor countries.
"If they were living in our neighbourhoods obviously we would reach out and help them and that's why today's event is so important."