Colossus was a project made too break the Lorenz cipher faster. The Lorenz cipher was a code Germans used in WWII that was created by the Lorenz Company under order by the German Army High Command. It was made to encipher teleprinter messages. Many scientists, including Alan Turing, worked on it but Max Newman, a mathematician, came up with the idea. The building of Colossus started in March 1943. It was called "the Mark I Colossus".

The Colossus even helps contribute to D-Day after it intercepted one of Hitler's messages that said that he was convinced that the attacks were coming across the Pas de Calais and that he was keeping the Panzer divisions in Belgium, therefore, the allies knew where to it would be safer to attack. After updating it a bit, the Colossus was able to break the cipher even faster. The newly upgraded machine was called "the Mark 2 Colossus". Eight more were built to help with the rush if messages they were getting.

The Colossus lived up to its name. Racks were 90 inches high of varying widths. There were eight racks arranged in two bays about 16ft long plus the paper tape reader and tape handler, which is known as the bedstead. The front bay of racks, spaced 5ft from the rear bay, comprised from right to left, the J rack holding the master control panel, the plugboard some cathode followers and the AND gates. Next came the K rack which contained the very large main switch panel together with the very distinctive sloping panel at the front which was a duplicate patch panel for the thyratron rings. After that the S rack which held the relays used for buffering counter output and making up the typewriter drive logic. The left hand rack at the front was the C rack which held the counter control logic on the front and the decade counters on the back.

The rear bay of Colossus contained four racks, the R rack holding the staticiser and delta boards for the paper tape reader output and the K and S-wheel thyratron ring outputs, the M rack for the M-wheel staticisers and S-wheel motion logic, the very large W rack, which held on one side all the thyratrons making up the wheel rings, 501 in all, and on the other side the 12 thyratron ring control panels, and the end rack of the back bay held the power packs. The circuit layout was all surface mounting on metal plates bolted to the racks. The valve holders were surface mounting with tag strips for the components. This form of construction had much to commend it, firstly both sides of a rack could be used, secondly wiring and maintenance were very easy and lastly cooling of the valves was expedited by them being horizontal.

The Colossus is important to the computer revolution because it was an amazing computing achievement that had never been seen before. Eight of the ten Colossi were dismantled after WWII. Two went to Eastcote in North London and then to GCHQ at Cheltenham. These last two were dismantled in about 1960 and in 1960 all the drawings of Colossus were burnt. Its very existence was kept secret.