Interest in the assessment and management of pain increased dramatically after 1965, when the gate control theory was introduced. This increase is concurrent with enormous advances in our understanding of the plasticity and complexity of pain processing. New information about internal pain-inhibitory systems and the factors that trigger them has revolutionized traditional approaches to pain control. This article describes some of the new information on the factors that influence human pain perception, specifically the environmental and psychological factors that modify how pain is experienced. Pain is a complex, multidimensional perception that varies in quality, strength, duration, location, and unpleasantness. The strength and unpleasantness of pain is neither simply nor directly related to the nature and extent of tissue damage. Even newborn infants may experience different pains from the same stimulus, because of the differences in the situations in which it is administered. Pain experiences can range from an inability to perceive pain, regardless of the strength of stimulation, to the actual perception of pain in a limb that has been amputated. The perception of, expression of, and reaction to pain are influenced by genetic, developmental, familial, psychological, social and cultural variables. Psychological factors, such as the situational and emotional factors that exist when we experience pain, can profoundly alter the strength of these perceptions. Attention, understanding, control, expectations, and the aversive significance can affect pain perceptions. Consequently, the understanding of pain requires not only understanding of the nociceptive system, but recognition and control of the many environmental and psychological factors that modify human pain perceptions.