Publisher Summary This chapter describes developments of biosensors for detecting organophosphate (OP) nerve agents. Electrochemical biosensors are devices that are fabricated by combining proteins or other biological molecules and electrodes. Among the biosensors developed so far, enzyme-based biosensors have been most extensively studied due to their high sensitivity and wide applicability in biomedical and environmental analyses. One of the merits of an enzyme biosensor is its versatility. The selectivity of biosensors directly relates to the type of enzyme used for constructing biosensors. The electrochemical biosensors used for detecting OP compounds can be divided into three types depending on the type of enzymes used for constructing biosensors: choline esterase (ChE)–choline oxidase (ChOx) bienzyme-modified biosensors, ChE-modified biosensors, and organophosphorus hydrolase (OPH)-modified biosensors. Different types of transducers other than electrodes have also been used for constructing OP biosensors. Researchers have found that thiocholine-stimulated growth of gold NPs is inhibited by OP nerve agents, depending on the concentration enabling the NPs to be used for detecting OP compounds. In addtion polymer microbeads are also used for constructing an OP detection system using fluorescence response of the microbeads.