Randell's research interest is the history of computing. He first started to pursue this interest actively soon after he reached Newcastle, prompted by his coming across the work of Percy Ludgate, the little-known Irish computer pioneer, while Randell was preparing material for his inaugural lecture. The main results of this research interest have been the book that Randell edited entitled The Origins of Digital Computers, and his unveiling of the Colossus machines-code breaking computers that were developed secretly in the UK during World War II.