The application of the electrochemical biosensors is demonstrated for the detection of toxins. Two categories of the toxins which are inhibitors to acetylcholinesterase (ACHE) enzyme are discussed. The first one is the reversible inhibitor that binds the enzyme based on competitive inhibition and the inhibited sensor is reactivated using the excess concentration of natural substrate for the enzyme. The second category includes those that form a covalent linkage with the active site of the enzymes and are referred as irreversible inhibitors. The reactivation in this case is carried out using a variety of oximes. The mechanism of enzyme inhibition and its relation to inhibitor concentration are discussed.