Tony Blair became Prime Minister when the Labour Party won the 1997 General Election. For the previous 18 years, the UK had been ruled by the Conservatives.
One of Mr Blair's first big public duties was speaking out on the death of Princess Diana the following August.
Leo Blair was the first child born to a serving Prime Minister for 152 years. He arrived in May 2000. Mr Blair and his wife Cherie have four kids.
Mr Blair is the first Labour leader to win three General Elections in a row. He won a massive majority in 1997, then saw his party returned to power in 2001 and again in 2005.
Mr Blair's decision to offer British military support to US President George Bush's invasion of Iraq in 2003 split his Government, his party and the country.
Four years after the invasion, British troops are still in Iraq. Here is Mr Blair on a 2006 visit to the country.
Since Mr Blair became Prime Minister, Scotland and Wales are governing themselves in some areas. And in Northern Ireland, politicians have agreed to start sharing power.
Mr Blair and his wife Cherie have three sons - Euan, 23, Nicky, 21 and Leo, who will be seven on May 20. They also have a daughter, Kathryn, 19.