Various reasons exist for the frequent failure of neurosurgical procedures to control pain. Three main classes of factors are involved in this failure--those relating to the surgeon and the surgery, those dependent on the disease, and those dependent on the nervous system itself. In the first class, technical and conceptual difficulties may explain the recurrence of pain after neurosurgery, including our incomplete understanding of the function of recognized pain pathways, the failure of the surgeon to apply known information, and technical limitations in our ability to carry out neurosurgery. Factors dependent upon the disease itself include alterations in the physical extent of the disease, and in the types of pain caused by it. Factors dependent on the nervous system include nerve and tract regeneration, changes in the anatomy of the CNS in response to both the disease and the neurosurgery, and, importantly, somatotopographic reorganization including the opening of alternative pathways.