This paper describes the evolution of the pathogenic concepts associated with the infection by the human immunodeficiency virus (HIV), with emphasis to the pathology of the nervous system. Although the first description of damage to the nervous system in the acquired immunodeficiency syndrome (AIDS) only appeared in 1982, the dramatic diffusion of the epidemic worldwide, as well as the invariably rapidly fatal outcome of the disease before the introduction of efficient treatment, generated from the beginning an enormous amount of research and re-thinking on a number of pathogenetic concepts. Less than 25 years after the first autopsy series on AIDS patients were published and the virus responsible for AIDS was identified, satisfactory definition and classification of a number of neuropathological complications of HIV infection have been established. This has led to the establishment of accurate clinical and biological diagnosis of the main neurological complications of the disease, which remain a major cause of disability and death in patients. Clinical and experimental studies have provided essential insight into the pathogenesis of CNS lesions and the natural history of the disorder. The relatively recent introduction of effective antiretroviral therapy in 1995-6 dramatically improved the course of prognosis of HIV disease. However, there remain a number of unsolved pathogenetic issues, the most puzzling of which remains the precise mechanism of neuronal damage underlying the specific HIV-related cognitive disorder (HIV-dementia). In addition, although antiretroviral therapy has changed the course of neurological complications, new issues have emerged, such as the lack of improvement or even paradoxical deterioration of the neurological status in treated patients. Interpretation of these complications remains largely speculative, partly because of the small number of neuropathological studies related to the beneficial consequence of this treatment.