Pain has been unrecognized and undertreated throughout the history of neonatal care. Misconceptions about the infant's ability to feel, remember, and express pain contribute to this long-standing problem. These misconceptions include beliefs that infants are unable to feel pain like adults or to remember it. This article describes the embryology of pain and includes a discussion of emerging evidence that infants do remember pain and consequently react differently to subsequent painful experiences. In addition, the adverse long-term effects of pain on the developing infant are identified and discussed. Permanent structural and function changes in the brain and spinal cord occur with repeated painful experiences, and adverse outcomes are described. An enhanced understanding of the infant's ability to experience pain and the long-term effects of unrelieved pain are essential to avoid harm and to maximize short- and long-term neurodevelopmental outcomes in this vulnerable population.