 Designing clinical trials requires careful selection of outcomes to compare interventions directly, and these outcomes must be relevant and important to key stakeholders for policy and practice influence. A core outcome set, an agreed standardized collection of outcomes, has been developed to address the issue of insufficient attention paid to outcome measurement in clinical trials. The Comet Initiative recommends a four-step process for developing a core outcome set, with the goal of updating the handbook as further research is identified. This article was offered by Paula R. Williamson, Douglas G. Altman, Heather Bagley, and others.