The major concepts presented in this review can be summarized as follows: 1. There is a multidirectional continuum of anesthetic states—some represented by CNS excitation and others by depression. 2. The reticular activating system is influenced by all anesthetics; some inhibit its action (stage III) and some hyperexcite the system resulting in a function disorganization (stage II-C). 3. Some agents traverse both excitation and depression, diethyl ether (I, II, III). 4. Others induce only stage II—catalepsia, e.g. nitrous oxide, ketamine, gamma-hydroxybutyrate, alpha-chloralose, phencyclidine, trichlorethylene, and enflurane. 5. Others induce no stage II but progress directly from stage I stage III, e.g. halothane and barbiturates. 6. Cataleptic agents may induce further CNS excitation manifested by seizures, e.g. gamma-hydroxybutyrate, phencyclidine, ketamine, alpha-chloralose, trichlorethylene, and enflurane. 7. The functional definition of surgical anesthesia is: a stage induced by a drug that makes the subject relatively unresponsive to painful stimuli and amnestic. Thus, the subject does not respond during surgery and cannot recall what happened afterwards. This state can be achieved by functional disruption of CNS systems by marked stimulation or depression.