This TIP, Medication-Assisted Treatment for Opioid Addiction in Opioid Treatment Programs, incorporates the many changes in medication-assisted treatment for opioid addiction (MAT) that have occurred over the most active decade of change since the inception of this treatment modality approximately 40 years ago. The TIP describes the nature and dimensions of opioid use disorders and their treatment in the United States, including basic principles of MAT and historical and regulatory developments. It presents consensus panel recommendations and evidence-based best practices for treatment of opioid addiction in opioid treatment programs (OTPs). It also examines related medical, psychiatric, sociological, and substance use disorders and their treatment as part of a comprehensive maintenance treatment program. The TIP includes a discussion of the ethical considerations that arise in most OTPs, and it provides a useful summary of areas for emphasis in successfully administering MAT in OTPs.