Collaboration is increasingly important for researchers in all disciplines. Universities and funding bodies tend to prefer projects that involve interdisciplinarity, collaboration between different institutions, and international consortiums. Such projects can yield great benefits, but they also pose particular challenges for certain aspects of research integrity, and particularly for awarding credit and authorship. In this article, we describe and analyze the phenomenon of the ghost collaborator, who is initially fully involved and makes a full contribution to a project's design, but then finds him- or herself excluded from meetings and publications.