The 17th/21st Lancers was a cavalry regiment of the British Army. It was formed in England by the amalgamation of the 17th Lancers and the 21st Lancers in 1922 and, after service in the Second World War, it amalgamated with the 16th/5th The Queen's Royal Lancers to form the Queen's Royal Lancers in 1993.

There are 13 cavalry Regiments of the British Army each with its own unique cap badge, regimental traditions, and history. Of the currently nine regular cavalry regiments, two serve as armoured regiments, three as armoured cavalry regiments, three as light cavalry, and one as a mounted ceremonial regiment. There are also four yeomanry regiments of the Army Reserve, of these, three serve as light cavalry and one as an armoured regiment. Each yeomanry light cavalry unit has been paired with a regular unit of the same role, the armoured yeomanry unit is paired with the two regular armoured units. All except the Household Cavalry are part of the British Army's Royal Armoured Corps.