Extracts of the opium poppy have been used for the relief of pain since antiquity. The active principle, morphine, continues to this day to be one of the most effective ways of alleviating moderate and severe pain. It is also one of the most addicting drugs known to man. This chapter describes the history of research on opium and the pharmacologic properties of morphine and related drugs. It briefly summarizes critical studies in the development of current knowledge of their antinociceptive actions; their effects on arousal, respiration, locomotor activity, and behavioral reinforcement; endocrine effects; and actions on peripheral tissues including the gastrointestinal tract, the genital tract, and the heart and circulatory system. The last section provides an overview of the absorption, metabolism, and distribution of morphine and other major opiate drugs.