The operational base and training establishment was HMS Varbel at the former Kyles Hydro Hotel at Port Bannatyne on the Isle of Bute in the Firth of Clyde, Scotland.

Their first deployment was Operation Source in September, 1943, an attempt to neutralise the heavy German warships based at Kåfjord, Nordkapp in Northern Norway.

Tirpitz was badly damaged, crippled, and out of action until May 1944; it was destroyed on 12 November 1944 by Avro Lancaster bombers during Operation Catechism in Tromsø, Norway.

[4] For this action, Cameron and Place were awarded the Victoria Cross, whilst Robert Aitken, Richard Haddon Kendall, and John Thornton Lorimer received the Distinguished Service Order and Edmund Goddard the Conspicuous Gallantry Medal.

[6] There was a possibility that X5 had also successfully planted explosive side charges before being destroyed, but this was never conclusively proven; its commander Henty-Creer was not awarded a medal, but was mentioned in dispatches.

Operation Postage Able was planned to take surveys of the landing beaches with X20, commanded by Lt KR Hudspeth, spending four days off the French coast.

On the third night, they were due to go ashore off the Orne Estuary (Sword Beach), but by this stage fatigue (the crew and divers had been living on little more than benzedrine tablets) and the worsening weather caused Hudspeth to shorten the operation, returning to Dolphin on 21 January 1944.

The craft were also equipped with a radio beacon and echo sounder to help direct Canadian and British ships to the suitable positions on Sword and Juno beaches.

This type of midget submarine was portrayed in the 1955 war film, Above Us the Waves, featuring John Mills, which was based on both Operation Source and the earlier Chariot attacks on the Tirpitz.

A 1976 Douglas Reeman novel, Surface with Daring, features a fictionalized account of X-class midget submarines, especially XE-16 and its crew, performing several highly secret operations in occupied Europe.

[22] A 2006 Alexander Fullerton novel, The Gatecrashers, features a fictionalized account of X-class midget submarines, including X-12 piloted by one of the protagonists, that lays explosive charges to damage the Tirpitz.