Enzyme-substrate recognition provides a convenient and powerful basis on which to construct molecular sensors. In direct enzyme electrodes the rate of the enzyme reaction is transduced into a current using an electrode made of a conducting organic salt. In vivo microelectrodes designed to measure glucose have been constructed and used in the brain of the freely moving rat. Another strategy is to use enzymes that operate with NADH; the NADH can readily be oxidized on conducting organic salt electrodes. Results for the measurement of micellar equilibria involving bile acids are presented. The packed-bed wall-jet electrode provides a device of greater sensitivity; results for the measurement of femtomoles of acetylcholine obtained by microdialysis from cerebral fluid demonstrate the power of this method. The wall-jet ring-disc electrode can be used in an electrochemical immunoassay again at the femtomole level. Finally, enzyme inhibition can be used to make a sensor for toxic substances such as H2S at the p.p.m. level.