The large number and diversity of anesthetic agents were evident to investigators 80 years ago, and suggested a physicochemical theory of anesthesia. Meyer and Overton were the first to offer a quantitative relationship between a physicochemical property and potency of anesthetic agents. They also focused attention on the lipid phase as the site of anesthetic action. Ferguson realized that the concentration of an agent at its site of action bears a generally unknown relation to the concentration in the external phase. However, at equilibrium the activity of an agent is the same in every phase, motivating Ferguson to suggest that activities rather than concentrations be used as indices of dosage. The critical-volume theory resulted from modification of the Meyer-Overton theory to include the molal volume of the anesthetic. The allowance for molal volume resulted initially from an attempt further to regularize the experimental data. The concept of a critical-volume fraction of anesthetic being necessary for narcosis was discussed in most detail by Mullins. Subsequently, the concept of the effect of the anesthetic has changed from filling of free space to expansion and fluidization of the membrane. The ability of pressure to cause excitant phenomena and antagonize anesthetics is predictable from the critical-volume theory and is therefore highly significant evidence. K. W. Miller and associates are perhaps most prominent in the recent quantification and formalization of the critical-volume theory and HPNS. The existence of a separate convulsant site(s) is suggested by the demonstration of significantly different compressibilities associated with anesthesia and convulsions. Work corroborating a separate convulsant site involved measurement of the partial molal volumes of a series of related convulsant and anesthetic ethers and calculation of each compound's solubility parameter. Multiple convulsant sites may exist, and these two methods may not have accessed the same site. Understanding the anesthetic-convulsant duality will have important practical application to deepwater diving, and may well offer important insight into the neurophysiologic and electrophysiologic effects of anesthetics. The application of ESR and NMR allows investigation at the molecular level of effects of anesthetics on biological and model membranes. Magnetic resonance techniques have generally supported the concept of membrane fluidization by anesthetics. Some investigators have recently attempted to displace the focus of attention from the lipid phase. However, the evidence is clearly against the aqueous-phase theory of Pauling and S.L. Miller. The microtubule theory of Allison and Nunn has not accumulated supporting evidence comparable to the lipid theories. Contradictory evidence makes any evaluation of this theory speculative. Additionally, the interspecies and intracellular variability of microtubules raises questions of the relevance of many studies...