Evidence is presented that the signal that damage has occurred to an animal begins with the activation of receptors which respond specifically to noxious stimuli. In fact, different types of nociceptors are found which respond selectively to different types of damage. The activity of nociceptive sensory fibers influences neurons in the spinal cord which are not activated by other types of somatic stimuli and are thus specific. At higher levels of the nervous system less is known about the physiology of pain and such fundamental questions as the degree to which the cerebral cortex is involved in pain perception have not been answered. It is not known to what extent the mechanisms at higher levels are specific and the significance of convergent systems in which an individual neuron can be excited by a number of different stimuli, both noxious and innocuous, has not been resolved. However, it is argued that the evidence at present most strongly supports the concept that the neural system involved in pain is specific; the activity of neurons in this system either causes pain, or if the level of activity is insufficient, no sensation. Ways in which the activity of this specific system may be modulated are discussed in the context of counterirritation and acupuncture analgesia.