 This study found that post-approval studies are rarely used in relative effectiveness assessments conducted by four European HTA organizations. These studies are often not available when the initial assessment is made, and when they are available, they are usually used to change the conclusion of the assessment. There is currently no alignment between regulators and HTA organizations regarding the use of post-approval studies, which limits their potential. This article was authored by Rick Averyman, Lawrence T. Blum, Stain Van Ourschot, and others.