Causalgia, reflex sympathetic dystrophy, and sympathetically maintained pain (SMP) are a complex group of disorders, with symptoms of spontaneous/stimulus-induced pain and vasomotor, sudomotor or skeletomotor dysfunction of the involved area. Sympatholysis has been recommended for diagnosis/classification and treatment of these patients. Lack of adequate placebo control makes the physiologic response to this intervention unclear. Sensitization of wide dynamic range (WDR) neurons in the central nociceptive pathway has been proposed as a key element in pathophysiologic mechanisms of these disorders. Low threshold mechanoreceptors and nociceptors have been implicated as the primary afferents transmitting signals to or maintaining sensitization of WDR neurons in SMP. Vasomotor disturbances may result from antidromic vasodilatation, vasoparalytic dilatation, normal somatosympathetic reflexes, and denervation supersensitivity. There is conflicting information regarding the use of phentolamine and clonidine in these pain syndromes. Treatment of these patients remains a challenge given the many potential underlying mechanisms.