Although the exponential growth of the Internet has made it easier than ever to carry out plagiarism, it has also made it much easier to detect. This paper gives an overview of the many different methods of detecting web-based plagiarism which are currently available, assessing practical matters such as cost, functionality and performance. Different types of plagiarism detection services are briefly outlined by broad category. The paper then considers the relative advantages and disadvantages of the different methods, referring to comparative studies where possible. It also draws out some of the more general drawbacks of electronic detection, ranging from practical matters such as technical restrictions, data protection issues, and cost, to the human impact on staff and students alike. It seeks to counterbalance these drawbacks by outlining the many possible benefits of implementing online detection in academic institutions, aside from the obvious saving of time when dealing with large cohorts of students. It argues that if online detection is used in conjunction with the many valuable "anti-plagiarism" resources and tutorials available on the web, it really can become a positive teaching aid for staff and students alike, rather than a threatening online policing system. The paper concludes with a brief forecast for the future of plagiarism detection, and emphasises that any form of online detection service can only act as a mere diagnostic tool to highlight possible cases of plagiarism, with human judgement always needed to investigate further.