Plagiarism by students is seen as an increasing problem. The fear is that students will use the internet to obtain analysis, interpretation or even complete assignments and then submit these as their own work. Electronic plagiarism detection services may help to prevent such unfair practice but, in doing so, they create a new problem: certifying the absence of plagiarism. This article reports the results of an evaluation of one such service within an interdisciplinary school of social sciences. The article describes how the system works and the experiences of staff and students in using the service, together with an evaluation of the data generated. The key findings are that the service did identify examples of poor scholarship and unfair practice that had been missed under the usual marking system but that rigorously checking every script for plagiarism was impractical. Trust and student honesty thus remain central to a successful academic system. (Contains 2 tables.)