In the context of the rise of proactive and intelligence led policing strategies this article examines claims as to the cost effectiveness of the police use of informers. In particular it questions the Audit Commission's limited view of effectiveness based solely on reward money paid out and seeks to account for the full costs of running informers. These, it is argued, should include all operational and organisational costs associated with recruitment and managing informers. Moreover, as the reality of using informers raises profound ethical problems there is a danger that an increase in their use could further undermine police legitimacy.