This chapter considers the UK Parliament's use of external evidence in the scrutiny of policy and legislation. Throughout the nineteenth and most of the twentieth century, Members of Parliament (MPs) drew on their professional experience outside of Parliament to provide informed scrutiny of government policy and legislation. Since the latter part of the twentieth century, however, there has been a significant increase in opportunities for Parliament to draw on external evidence. Today, external evidence occupies a central place in Parliament's scrutiny and legislative functions. The chapter first examines how select committees scrutinize policy and administration, making a distinction between written evidence and oral evidence, before discussing the impact of evidence-taking on the legislative process for draft bills that are subject to scrutiny by public bill committees. It also describes formal mechanisms by which evidence and expertise are drawn into Parliament.