It's going to be the biggest parade in Windsor for a hundred years. Two and a half thousand members of the Armed Forces will be parading in front of The Queen tomorrow in honour of her Diamond Jubilee.
Thousands of people will be lining the streets and, in the skies overhead, an 86-plane flypast. It's an unprecedented event - and launches the Jubilee celebrations in style.
But such a major parade requires everyone to march to the same beat. So to ensure that everyone toes the line the RAF, the Navy and the Army have been practicing in Buckinghamshire.