This article examines the role of external inspection in enhancing the quality of a criminal justice system. It seeks to answer six foundational questions: how should we understand the nature and purposes of criminal justice inspection? what methodologies ought it to employ? who should do it? what values should it respect? how much does it cost? and does it ‘work’? The article reveals that the difference between inspection and other forms of scrutiny activity is largely a matter of emphasis; that the same is true of the difference between inspection and research; that ‘lay’ involvement in inspection can be beneficial; that independence is a core value for inspection, albeit one that is best understood as independence of judgement; that transparency is a further key value but not always honoured; and that evidence that inspection improves service delivery and hence justifies its costs is weak and further research is needed.