Miss Middleton, who comes from a wealthy middle class background, was born a commoner but is now in line to become Queen.
She was dubbed "Waity Katey" for her patience in biding her time before the Prince popped the question.
William was frequently asked by members of the public when he would get married, but his stock response was "wait and see".
University Years
Miss Middleton was credited with persuading the Prince to continue his studies at St Andrew's when he considered quitting as he struggled to adjust to university life during his first year.
They moved into a shared house with two other friends in 2002 for their second year, and were rumoured to have become an item around Christmas 2003.
The couple briefly split up in April 2007, reportedly because of the strain caused by the Prince's military career, but they were secretly back together within weeks.
She was privately educated at the £15,000-a-year Marlborough College, where she was a keen netball and hockey player, and has two younger siblings, Pippa and James.
Miss Middleton, who works for her parents' company, has kept a low profile for the past year and has only been seen at William's side at weddings or on holiday.
The announcement of the engagement will lead to speculation about when she will take on formal Royal duties.
Kate's parents
Miss Middleton's millionaire parents Michael and Carole, a former air hostess, run a mail-order business called Party Pieces which sells toys and party paraphernalia.
They said they were "absolutely delighted" and "thrilled" at today's announcement.
Miss Middleton's father Michael Middleton read a statement on behalf of the couple outside their home near the Berkshire village of Bucklebury.
He said: "I would just like to say that Carole and I are absolutely delighted by today's announcement and thrilled at the prospect of a wedding some time next year."
"As you know Catherine and Prince William have been going out together for quite a number of years which has been great for us because we have got to know William very well. We all think he is wonderful and we are extremely fond of him."
"They make a lovely couple, they are great fun to be with, and we've had a lot of laughs together. We wish them every happiness for the future."