Inhalation agents represent a basic drug used in modern balanced anesthesia. In the present review, the pharmacokinetics, effectiveness and clinical effects of inhalation agents on different systems are discussed. Data concerning the metabolism and related toxicity of halogenated agents is reviewed, with particular regard to the problem of chronic exposure to traces of anesthetic gases in the operating room. The cardioprotective effect of halogenated agents and the actual role of nitrous oxide and xenon are discussed. The different mechanisms of action of the inhalation agents and the evolution from a unitary theory of inhaled anesthetics to a multiple mechanism concept are presented.