The environment has profound influences on human health. Environment is the combination of natural (physical, chemical, biological) and cultural (sociological) conditions in which living organisms, man in particular, develop. Adaptation is the human physiological response to external factors including mechanisms such as circadian rhythms (sleep and wakefulness), biorhythms, thermoregulation, and others to adjust to changes in natural conditions (night and day, cosmic rhythms, climatic changes). Man remains vulnerable due to a number of factors: genetic, physiological, age, sex, impaired reparative or protective mechanisms, or acquired factors (risky behaviors and lifestyle, nutritional habits). Hazard is the potential of a particular factor to have a negative impact on health. Risk is the probability of that hazard occurring; it defines and measures the predictability of that hazard. A number of environmental factors have a profound influence on the pathogenesis of neurological disorders. Environmental neurology follows the classical diagnostic precepts: from symptom to syndrome in the search for etiology and individualized treatment of the patient. It also utilizes principles of epidemiology and public health, toxicology and occupational medicine; i.e., an approach by "milieu" and by specific factors or "agents." Environmental neurology offers a promising new field of practice and research.