Pain is among the most common complaints for which people seek medical care; yet pain is also among the most undertreated patient complaints. Reasons for this include reluctance by clinicians to prescribe and support the use of opioids, often due to a fear of addiction. To address this issue, three major health professional organizations that deal with the treatment of pain and addiction, the American Academy of Pain Medicine, the American Pain Society, and the American Society of Addiction Medicine, formed the Liaison Committee on Pain and Addiction (LCPA). The first mission of the LCPA was to formulate precise definitions of the terms addiction, physical dependence, and tolerance. This report explains these definitions and discusses how they apply to clinical practice.