Acetylcholinesterase (AChE; EC 3.1.1.7) is a primary target of many insecticides including organophosphates (OP) and carbamates (CB). Because AChE is expressed in all invertebrate and vertebrate animals as a key enzyme of the cholinergic system, the toxicity of anticholinesterase insecticides to mammals and non-target species such as beneficial insects has been a great concern. In addition, the intensive use of OP and CB insecticides has resulted in the development of resistance in many insect pests, which has limited the use of anticholinesterase insecticides. Many aces encoding AChEs have been sequenced from a variety of vertebrates, insects and other invertebrates, and crystal structures of four AChEs have been determined in the past 20 years. Although the primary motifs and the three dimensional (3D) structures of different AChEs are similar, differences among AChEs are obvious. The catalytic properties and inhibition kinetics of AChEs from different groups of insects and mammals may be quite different, and two AChEs from a single insect may also show distinct differences. These differences may provide new opportunities for designing more selective insecticides for pest management.