Progressive degeneration of functionally related groups of neurons occurs in certain infective, toxic, nutritional and genetically determined neurological diseases. It also takes place in normal aging, and several of the regions that undergo selective decay with the passage of time seem to be the same target regions that are afflicted in degenerative disorders such as Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis (ALS). Infective etiology is relatively easy to exclude by a combination of immunological tests and transfer experiments. Genetic causation can be rendered unlikely when large kindreds are available for study. Nutritional deprivation and acute or subacute toxicity are accessible to explanation by examining the environment. The most difficult mechanism of pathogenesis to refute is chronic toxic damage, where the lesion may derive from long-term exposure to a relatively widespread noxious agent or agents. Variations in involvement of individuals within a population may stem from differing capacities to activate or inactivate a toxin. Inherent in this concept of etiology is recognition that compensatory potential within the central nervous system may contribute to prolonged existence of subclinical lesions so that a latent period may exist for several decades, between causal event and the onset of symptoms. Furthermore, progressive clinical deterioration may take place even though the cause may have been transient, many years before. The histological features associated with Parkinson's disease, Alzheimer's disease and ALS may be nonspecific indicators of neuronal “illness”, there being a predilection for certain morphological markers to appear more frequently in particular circumstances and particular regions associated with the pathology of particular diseases.