This chapter explains the pharmacological effects of the anticholinesterases, which are quite widespread and reflect the extensive locations of Ach as a transmitter in skeletal muscle, autonomic ganglia, and parasympathetic nerve endings in the effector organ and in the central nerve system (CNS). The asymmetric form of the enzyme is associated only with innervated tissues and is found in muscles, the CNS and peripheral nerves. Cholinesterases hydrolyse cholinesters specifically and rapidly by an acylation–deacylation reaction. There are two distinct cholinesterase, acetyl cholinesterase and butyrylcholinesterase, the fundamental difference being their substrate specificities. Drugs which inhibit cholinesterase may be divided into two types based upon the nature of their interaction with the enzyme. They are reversible anticholinesterases and irreversible anticholinesterases. The prototype reversible anticholinesterase is physostigmine, a naturally occurring alkaloid obtained from the Calabar or Ordeal bean, the dried seed of Physostigma venenosum. The irreversible anticholinesterases are the organophosphorus compounds which usually interact only with the esteratic site.