The departmental select committees of the British House of Commons deal with executive oversight, but not with legislation. This has two important knock-on effects. First, although these committees are permanent, expert, and largely well regarded in Britain, they are often overlooked by comparative scholars. Second, and connectedly, their impact on policy is very difficult to assess. Even in the UK there is considerable scepticism about whether the committees really matter to policy outcomes. This paper reports the first detailed cross-departmental investigation of the select committees' impact on government policy for many years, employing both quantitative and qualitative methods. It concludes that numerous committee recommendations are implemented by government, including many for major policy change. But counting successful recommendations is a poor proxy for committee influence overall, and this also takes many other, perhaps more important, forms.