In clinical studies, beneficial and harmful effects of interventions are investigated by measuring predefined outcomes. The uncontrolled choice of these outcomes carries a risk of outcome reporting bias, large heterogeneity of outcomes with reduced options for evidence synthesis. Furthermore, such outcomes may not be important for stakeholders including patients. A potential solution to these problems is to develop a core outcome set (COS) - a list of outcomes which have been estimated to be important to include in all future studies in a given setting by all relevant stakeholders. This review summarises the benefits of defining a COS.