The theft of someone's words or thoughts--plagiarism--has long been a concern in medical literature. The phenomenon applies to unreferenced published or unpublished data that belong to someone else, including applications for grants and a publication submitted in a different language. Other acts of plagiarism are paraphrasing without crediting the source, using "blanket" references, "second-generation" references, and duplicate or repetitive publication of one's own previously published work. Does incorporating a peer reviewer's ideas constitute plagiarism? The requirement of many journals for a short list of references is problematic, as is confusion about what constitutes common knowledge. What criteria should be used for detecting plagiarism? To make an accusation of plagiarism is serious and perilous. Motivations for plagiarism are considered, and 2 striking historical examples of plagiarism are summarized. We believe that with insight into its causes and effects, plagiarism can be eliminated.