The operation the Duke of Edinburgh will undergo on Wednesday will require a general anaesthetic.
It's not unusual to do that to someone who is 96 years old, but it does involve higher risks than for a patient of a younger age.
Prince Philip was taken to hospital on Tuesday ahead of the planned surgery.
With elderly patients, medics sometimes opt for an epidural and sedation rather than a general anaesthetic.
Buckingham Palace have not given any further details of the procedure, other than to say the operation is on his hip.
Most surgeons suspect the Duke is having a hip replacement operation – which is quite common for people of his age.
In fact, the Queen Mother had one of her hips replaced when she was older than her son-in-law.
She fully recovered from the operation at the age of 98, but that procedure was carried out after she broke her hip.
Wednesday's elective surgery was planned as the Duke has been suffering from pain with his hip for the last month or so.
It was the reason given for him missing both the Maundy Thursday service and the Easter Sunday service at St George's Chapel in Windsor.
He also didn't attend a ceremony earlier in March to mark the appointment of his son, Prince Andrew, as Colonel of the Grenadier Guards.
The Duke of Edinburgh had held that position until his retirement last year.
Prince Philip has been in good health for many years, but he was admitted to the same hospital - King Edward VII in central London – last June when he was understood to have been suffering from a bladder infection.
Both the Duke of Edinburgh and the Queen had been staying at Windsor Castle. Royal Court traditionally moves there for the Easter holidays.
The Royal Standard – the flag which is flown to show the Queen is in residence – is still flying at Windsor.
It is not expected that the Queen will visit her husband in hospital if his stay there is a short one.
However, the Queen Mother spent nearly three weeks in hospital after her first hip replacement operation in 1995.