BACKGROUND: Since its inception in 2006, the Institute for Clinical and Economic Review (ICER) has rapidly gained influence on drug pricing and reimbursement decisions despite historical resistance to the use of cost-effectiveness thresholds in the US health care system. Although patient groups, physicians, and pharmaceutical manufacturers voiced their concerns about the potential negative effects of increased use of ICER's assessments on patient access to innovative medications, there is little guidance and consensus on how the stakeholders should collaborate with ICER to ensure that its reviews reflect the best clinical and economic evidence. OBJECTIVES: To (1) summarize the evolution of ICER's evaluation procedure, scope, and topics; (2) evaluate the effectiveness of stakeholder engagement approaches; and (3) inform stakeholders of their potential role in collaborating with ICER in estimating the cost-effectiveness of new interventions. METHODS: Publicly available ICER evaluations from 2008 to 2019 were systematically reviewed. Changes in evaluation procedures, scope, and topics were summarized. For evaluations that occurred in 2018 (n = 12) and 2019 (n = 8), key characteristics were extracted from 172 letters documenting interactions between ICER and all stakeholders who provided comments to draft reports. Stakeholder suggestions were analyzed in terms of their effectiveness indicated by ICER's reconsideration of its original cost-effectiveness analysis approach. RESULTS: The number of ICER evaluations increased consistently from 2 to 12 per year between 2008 and 2018 but declined to 8 in 2019. Stakeholder opportunity to engage with ICER increased from 1 to 3 per evaluation between 2008 and 2015. ICER initially focused on reviewing general treatment strategies but shifted its focus to specific pharmaceuticals and medical devices in 2014. In 2018 and 2019, 30% of 172 stakeholder letters resulted in a revision in the base-case analysis (49 comments in 2018, 23 in 2019); nearly half of comments in these letters included specific alternative data or a published article to rationalize recommendations. Other common types of suggestions that resulted in ICER's base-case analysis revisions included comments relating to inconsistent methods used to derive model inputs across different treatments (12/49 in 2018, 5/23 in 2019); clinical justifications (12/49 in 2018, 0/23 in 2019); and justifications based on patient perspectives (1/49 in 2018, 5/23 in 2019). These revisions rarely affected ICER's conclusions on the cost-effectiveness of evaluated interventions. Among the 20 assessments that involved 172 stakeholder engagements in 2018 and 2019, only 2% (n = 3) of the engagements (all from 2018) were associated with a change in the cost-effectiveness conclusion. CONCLUSIONS: Between 2018 and 2019, stakeholders leveraged ICER evaluations as opportunities to promote dialogue for better understanding of the value of technologies. Actionable, evidence-based recommendations were accepted more often than other recommendations. DISCLOSURES: No outside funding supported this study. The authors have no conflicts of interest to disclose. Findings from this study were presented as a poster at Virtual ISPOR, May 17-20, 2021.