This is the first account of the relationship between select committees and the National Audit Office (NAO). The relationship has been often dismissed, but this article argues that it has been vital for the success of the committee system. Initially, both the committees and the audit office were established to strengthen the scrutiny of public administration. Despite the intentions of the early reformers, they did not initially collaborate. In the 1990s and 2000s, early reluctance gave way to enthusiastic collaboration between the two institutions. Initially, this collaboration focussed on financial and environmental scrutiny, but it has since expanded as the auditors have gained more confidence and the committees more familiarity with their work. Now the NAO advises committees on subjects such as European Union exit, which it would have never discussed with them only 40 years previously. This article provides both an account of that collaboration and an explanation for why the NAO and the committees have become so closely associated.