The U.S. opioid crisis worsened dramatically with the arrival of synthetic opioids, such as fentanyl, which are now responsible for tens of thousands of deaths annually. This crisis is far-reaching and even with prompt, targeted responses, many of the problems will persist for decades to come. RAND Corporation researchers have completed numerous opioid-related projects and have more underway for such clients and grantors as the Agency for Healthcare Research and Quality, the Assistant Secretary of Health and Human Services for Planning and Evaluation, the National Institute on Drug Abuse, the White House Office of National Drug Control Policy, and Pew Charitable Trusts. Researchers have advanced an understanding of the dimensions of the problem, some of the causes and consequences, and the effectiveness of different responses. However, no one has yet addressed the full scope of the problems associated with opioid use disorder and overdose deaths.