Several Australian universities are proposing to introduce use of plagiarism-detection services, specifically turnitin.com, for checking student essays. Having studied plagiarism issues for over 20 years, I decided to look at educational rationales for using such services, especially (1) deterring and detecting cheating, and (2) fostering learning of proper acknowledgement practice. A wider treatment would also cover implications for workloads, intellectual property and institutional reputation. Plagiarism involves claiming credit for ideas or creations without proper acknowledgement. In an academic context, acknowledgement is typically given in the form of citations or explicit statements of thanks. This is important for several reasons, including to give credit for ideas or words, to provide support for one's argument, and to show that one is aware of sources. To speak of proper acknowledgement is to focus on the positive side of scholarly practice; to speak of plagiarism is to focus on the negative. In most cases, software for detecting plagiarism can detect only word-for-word plagiarism for those documents in its database. It cannot detect plagiarism of ideas or plagiarism of authorship unless they also involve detectable word-for-word plagiarism. Students who take ideas from others but express them in their own words will not be detected. Nor will students who purchase custom-written essays. Nor will those who copy from sources not on detection databases, such as many printed texts, CD-ROMs, certain subscription databases and the deep web, or who use translations of documents.