The paths that took men and women from their ordinary lives and deposited them on the doorstep of the odd profession of cryptanalysis were always tortuous, accidental, and unpredictable. The full story of the Colossus, the pioneering electronic device developed by the Government Code and Cypher School (GC & CS) to break German teleprinter ciphers in the Second World War, is fundamentally a story of several of these accidental paths converging at a remarkable moment in the history of electronics—and of the wartime urgency that set these men and women on these odd paths. Were it not for the wartime necessity of codebreaking, and were it not for particular statistical and logical properties of the teleprinter ciphers that were so eminently suited to electronic analysis, the history of computing might have taken a very different course. The fact that Britain’s codebreakers cracked the high-level teleprinter ciphers of the German Army and Luftwaffe high command during the Second World War has been public knowledge since the 1970s. But the recent declassification of new documents about Colossus and the teleprinter ciphers, and the willingness of key participants to discuss their roles more fully, has laid bare as never before the technical challenges they faced—not to mention the intense pressures, the false steps, and the extraordinary risks and leaps of faith along the way. It has also clarified the true role that the Colossus machines played in the advent of the digital age. Though they were neither general-purpose nor stored-program computers themselves, the Colossi sparked the imaginations of many scientists, among them Alan Turing and Max Newman, who would go on to help launch the post-war revolution that ushered in the age of the digital, general-purpose, stored-program electronic computer. Yet the story of Colossus really begins not with electronics at all, but with codebreaking; and to understand how and why the Colossi were developed and to properly place their capabilities in historical context, it is necessary to understand the problem they were built to solve, and the people who were given the job of solving it.