In 2023 the Modern Records Centre (MRC) celebrated its fiftieth anniversary. Since its opening in 1973 it has become Britain’s leading archive for national trade-union and employers’ association records, and holds significant material relating to fringe politics, pressure groups, higher education, and cycling. This article charts the MRC’s early development, and discusses the historical and research climate at the time of its establishment and the initial controversy over its role in the archive world. It studies four of the MRC’s collections – the National and Local Government Officers’ Association, the Howard League for Penal Reform, Fakenham Enterprises Ltd, and the Disablement Income Group – showing the range of topics covered in the archive’s early deposits and the ongoing relevance of these today. Along with how and why these collections came to the MRC, the article reflects on the sorts of records contained within them and what this means for those who use them.