Background: There is growing evidence that overdiagnosis presents an important health challenge, along with associated problems of overmedicalisation and overtreatment. Emerging evidence suggests the problem of overdiagnosis may be highly significant—not only in the screening setting, but also in clinical settings—across a range of diseases. There is a growing debate about the best ways to address its causes—which include expanding disease definitions and more sensitive diagnostic and prognostic tests—and how to advance solutions. The role of good evidence, either from primary studies or systematic reviews, will be vital in this debate. As one of the most trusted sources of high quality evidence internationally, the Cochrane Collaboration, its review groups and/or reviewers, could play important roles in helping to understand the nature and extent of the problem and how to best address it. Potentially the Collaboration could be engaged in more reviews of the evidence about overdiagnosis and/or overtreatment, either in reviews of therapeutic interventions, diagnostic tests and prognostic factors or models. More broadly, other types of reviews—for instance reviews of studies that seek to define disease boundaries—could help to provide reliable evidence on which to base more appropriate diagnostic criteria and disease definitions. Structure of the session: The session will open with two or three short presentations, and then move through a structured interactive discussion and debate with those attending, with one focus being potential recommendations for systematic reviews of diagnostic tests and of therapeutic interventions. Chair and panellists: The session will be chaired by Professor David Henry, Chief Executive Officer of the Institute for Clinical Evaluative Sciences, Toronto, Canada, with brief presentations from Ray Moynihan, Senior Research Fellow, Bond University, and a co-organiser of the 2013 Preventing Overdiagnosis conference, and Karel Moons, University Medical Centre Utrecht, The Netherlands.

Target audience and level of expertise: The audience could be any of those attending the Cochrane Colloquium—including clinicians, researchers, reviewers, policy-makers, consumer/citizen advocates and others interested in the problems of overdiagnosis and overtreatment.