Comparative Effectiveness Research (CER) is the comparison of two or more health care interventions and the evaluation of the outcomes associated with these interventions in real-world clinical practice. The comparison can be among multiple therapies, such as two competing medications, comparison of a medical versus a surgical intervention, or an evaluation of how new therapies are introduced and used in community practices. Overall, CER is applicable to evaluation of competing therapies, health care practices, clinical guidelines and behavior modification. Study designs used for randomized clinical trials have been developed for narrower purposes, and may not be applicable for CER which must evaluate an array of interventions and outcomes for patients with diverse characteristics. ICON is experienced in the many approaches for designing and executing these studies, including pragmatic randomized trials, retrospective studies, disease registries, decision models, systematic literature reviews and health technology assessments.