The early discovery of AChE inhibition by OP compounds has led to their development primarily as insecticides, but also as nerve agents and to a limited extent, as drugs. OPs are potent and effective insecticides and still represent the largest group of insecticides sold worldwide. Given the strong similarities of the insect and mammalian cholinergic nervous system, these compounds are, however, responsible for the million of poisonings and thousands of deaths occurring annually as a result of pesticide exposures, particularly in third world countries. Certain OPs, in particular those containing isopropoxy substituents, have also been developed, and some have been used, as nerve agents for chemical warfare. Examples of such compounds are soman, sarin, tabun and VX. A few OPs were also developed as pharmaceutical drugs. Examples are metrifonate (trichlorfon), used on anthelmintic in schistosamiasis, or echothiophate, used for the treatment of glaucoma. However, use of AChE inhibitors for treatment of these and other conditions (e.g., myasthenia gravis, termination of the effects of competitive neuromuscular blockers, Alzheimer's disease) relies mostly on carbamates or other compounds [10].