In university plagiarism policies, and in the research into plagiarism, one form of collusion remains virtually unacknowledged: substantive editing performed by editors. While almost all Australian universities allow postgraduate students to have their thesis professionally edited, "substantive" editing is prohibited. This article discusses the problems inherent in this arrangement, given that most students and supervisors, and some editors, are not clear on where the line is between the two. Moreover, it is evident that there are a myriad of social and economic pressures that lead to substantive editing being performed even when all three parties are aware that it is occurring. Substantive editing is a particularly insidious form of plagiarism since it has received so little attention by universities, it is seen as less serious than other forms of plagiarism, it requires high levels of vigilance to detect and there are no deterrents in place. This article argues that there are significant numbers of students engaging in this form of plagiarism. This study aims to shed light on this issue by examining the nature, motivations behind and prevalence of substantive editing, while recognising the social context and negative effects of the issue.