The negative connotations of plagiarism as an illegal appropriation of ideas are based on the concept of Intellectual Property. Although Intellectual Property Laws in most countries around the world are specific as to the characterisation of plagiarism as an offence, the extent of plagiarism litigation varies enormously and this variation has a lot to do with the way writers, whose texts are plagiarised, and plagiarists themselves, view the act of being plagiarised or the act of plagiarising somebody else’s text. In countries which fall within the Common Law tradition such as the United States, Australia, Canada, Great Britain, plagiarism litigation is extensive and there is a regular offer of linguistic expertise to solve plagiarism disputes. In countries within the Civil Law tradition, like Spain, for example, linguists are still rarely called upon as expert witnesses in plagiarism cases. Plagiarism is multidimensional, as is proved in the number of areas of knowledge affected by it (including literature in all its forms: essay, novel, theatre, poetry), the settings and activities in which it occurs (education, translation), and the contexts in which it is produced (for example, the scope of plagiarism on the Internet is twofold since one can plagiarise directly from the web or use the web as a method to detect plagiarism). As expert witnesses, linguists are frequently asked to give evidence in court to help to decide cases of plagiarism of ideas, linguistic plagiarism, or both. In the first case, the distinction between author’s rights and copyright may be useful, because these two concepts and terms are used differently in different judicial systems. In the second case, it may be important for linguists to come up with theoretical and methodological proposals that help them as legal consultants to find linguistic markers and discourse strategies that will be decisive in plagiarism detection, as well as in establishing prima facie cases. As in any other forensic linguistics contexts, plagiarism is an area where the need to incorporate internal and external validity to the experts’ findings is strongly felt. When giving opinions in court, it has been proven that both qualitative and quantitative approaches to plagiarism detection are valid and complementary, and also that both semantically and statistically expressed opinions may be necessary.