Although the anticholinesterase (anti-ChE) agents have only limited applica tions in therapy, and from the viewpoint of practical significance they are more appropriately classified as toxic compounds or insecticides than as drugs, in their capacity of pharmacological tools they have few equals. The concept of neuro humoral transmission was originally established largely from experiments in which physostigmine, or eserine, was employed to protect acetylcholine (ACh), the trans mitter of the cholinergic nerves, from rapid hydrolytic destruction by acetyl cholinesterase (AChE) and other cholinesterases (ChE's). Since then, a great num ber of additional reversible and irreversible anti-ChE agents also have been indis pensable in studies of synaptic and neuroeffector transmission, and of other physiological processes. At the same time, there is practically no other class of compounds for which a mechanism of pharmacological action can be described in such concrete biochemical and physiological terms. Consequently, it is not sur prising that a huge literature has developed on these several closely interdependent topics. The assembling anrl proper correlation of this material for the present volume has taken the collaborative efforts of over two dozen . investigators. It iR believed that their contributions to this end will prove invaluable to future in vestigators in providing a ready, inclusive source of established information, in defining areas where further studies are indicated, and in preventing unnecessary duplication of past work. How well these aims have been accomplished will be for time and the reader to judge.