One of the fundamental ethical concerns about biomedical research is that it frequently exposes participants to risks for the benefit of others. To protect participants' rights and interests in this context, research regulations and guidelines set out a mix of substantive and procedural requirements for research involving humans. Risk thresholds play an important role in formulating both types of requirements. First, risk thresholds serve to set upper risk limits in certain types of research (e.g., pediatric research that offers the participating children no prospect of clinical benefit). Second, risk thresholds serve to demarcate risk categories that streamline risk-adapted systems of ethical oversight (e.g., expedited or no prospective ethical review of minimal risk research). But although risk thresholds play such an important role in research governance, there is a need both to better define the existing risk thresholds and to delineate new thresholds in order to develop more risk-adapted systems of research oversight. The present paper examines the existing minimal risk threshold and the surrounding debates with the goal of deriving a systematic approach to setting thresholds of research risk.