This volume addresses some facets of the adverse actions of chemical agents on the central and peripheral nervous systems in developing and mature states. Some of the effects of these chemicals are short-lasting and rapidly reversible; others, especially those that cause structural damage to the nervous system, may result in permanent damage to the organism. The nervous system has several levels of vulnerability to toxic substances. Some substances perturb ion channels or synaptic mechanisms required for the orderly transfer of electrochemical information within the nervous system. Others disrupt sites required for the maintenance of cellular integrity, and these variably result in degenerative responses of neurons and myelinating cells. Further sites of vulnerability include the delicate neural vasculature and neurohumeral mechanisms responsible for physiological homeostasis. The science of neurotoxicology inevitably is a multidisciplinary endeavor, with contributions from biochemistry, physiology, morphology and behavior, to name a few. The challenge is to apply appropriate techniques to investigate neurotoxic phenomena. The first logical step in this analysis is to determine from the point of view of the nervous system the nature of the exposure. Is the chemical a single or multiple entity; is it metabolized; how does it gain access to neural tissue? Once these factors are understood, changes induced by the exposure can be described at various levels from the biochemical to the behavioral.