Pain is a distressing or unpleasant sensation in the body and, over the course of a lifetime, is frequently a signal of disease. Although it is the most common presentation to physicians for an underlying condition, pain is also complex and varied. In acute settings, the negative experience of pain can bring awareness to an individual of a damaging source, allowing protection of the affected body part while also conditioning the individual to avoid such sources in the future. Pain is frequently the reason for physician consultation in developed countries and can significantly impact functioning and quality of life. This entry outlines the different classifications of human pain, the beginning of pain perception in development, and overviews pain mechanisms, assessment, and management.