Colossus was the world's first electronic digital computer that was programmable. The Colossus computers were developed for British code breakers during World War II to help in the cryptanalysis of the Lorenz cipher. Without them, the Allies would have been deprived of the very valuable military intelligence that was obtained from reading the vast quantity of encrypted high-level telegraphic messages between the German High Command (OKW) and their army commands throughout occupied Europe. Colossus used thermionic valves (vacuum tubes) to perform Boolean operations and calculations.

Colossus was designed by the engineer Tommy Flowers to solve a problem posed by mathematician Max Newman at the Government Code and Cypher School (GC&CS) at Bletchley Park. Alan Turing's use of probability in cryptanalysis contributed to its design. It has sometimes been erroneously stated that Turing designed Colossus to aid the Cryptanalysis of the Enigma. Turing's machine that helped decode Enigma was the electromechanical Bombe, not Colossus.

The prototype, Colossus Mark 1, was shown to be working in December 1943 and was operational at Bletchley Park by 5 February 1944 An improved Colossus Mark 2 that used shift registers to quintuple the speed, first worked on 1 June 1944, just in time for the Normandy Landings. Ten Colossi were in use by the end of the war.
The cipher text was input via paper tape and the 2500 valves of Colossus would find the Lorenz machine chi-wheel settings.
The main units of Flowers' design were as follows.

A paper tape transport and photo-electric reading mechanism very similar to Heath Robinson's.

A coder and adder that simulated the Lorenz machine using thyratron rings, the Thyratrons provided 1-bit memory.

A logic unit that performed Boolean operations.

A master control that contained the electronic counters.

A printer.

The Colossus computers were used to help decipher teleprinter messages which had been encrypted using the Lorenz SZ40/42 machine—British codebreakers referred to encrypted German teleprinter traffic as "Fish" and called the SZ40/42 machine and its traffic "Tunny". Colossus compared two data streams, counting each match based on a programmable Boolean function. The encrypted message was read at high speed from the paper tape. The other stream was generated internally, and was an electronic simulation of the Lorenz machine at various trial settings. If the match count for a setting was above a certain threshold, it would be sent as output to an electric typewriter.

The Colossus was used to find possible key combinations for the Lorenz machines – rather than decrypting an intercepted message in its entirety.
The machine is seen here desiphering an actual message sent in 2007 from a 12 wheel Lorenz cipher.