The Duchess of Cambridge has gone to hospital to give birth.
She was taken to St Mary's Hospital in London early on Monday morning and Prince William is at her side.
The BBC's Nick Witchell says the long wait is nearly over but there was likely to be no more news until the official announcement of the birth.
Announcing the birth
When the baby is born the official announcement will be displayed on an easel in the forecourt of Buckingham Palace.
The Queen, senior members of the Royal Family, and the duchess's family - if they are not at the hospital - will be told about the birth first.
Then a royal aide will take a bulletin, signed by key medical staff, from the hospital to the palace under police escort.
Future king or queen
It will be the duke and duchess's first child and will be third in line to the throne, meaning whether it's a boy or girl, it will become king or queen ahead of Prince Harry.
This year the rules on royal succession rules were changed: a 300-year-old law had meant a boy would always get to the throne even if he had older sisters.
But the government now want to make sure that changes and that whether William and Kate's child is a boy or a girl, they would become king or queen.
The act has been approved in Britain, but all 15 other Commonwealth countries around the world - where the Queen is the Head of State - still need to approve it for it to become law. It does look likely to happen though, according to Prime Minister David Cameron.
What's in a name?
Lots of people are speculating on what the baby will be called.
The current favourite names are George for if it's a boy, and Alexandra if it's a girl.
It's reckoned baby fever could bring in as much as £240 million to the UK as people celebrate this summer.
So it's no surprise that factories are already making items to commemorate the birth.