Amperometric biosensors for various drugs and metabolites have been developed, based on immobilised enzymes in both aqueous and organic media. Chapter 1 serves as a general review of the field of biosensors, including both an overview of recognition methods together with the techniques involved in transduction. The remainder of the thesis is divided into four experimental chapters. The first of these describes the development of some amperometric enzyme electrodes for certain drugs and metabolites in aqueous solution. Various immobilisation methods, including use of polymers such as Nafion and Eastman AQ55D, gelatine membranes, entrapment of the enzyme within electropolymerised films, together with direct adsorption, have been employed. Discriminative permselective films were found to show great effectiveness in excluding interfering compounds commonly found in serum. Application to the clinical and in vivo fields are discussed. Limits of detection down to the jiM level were found. Fast wash times permitted application to flowing streams with frequencies of up to 200 samples per hour, with good precision. The second experimental chapter deals with the construction of biosensors for inhibiting compounds in the organic phase. A study of the kinetics of tyrosinase at the rotating disc electrode in non-aqueous media is also described. Reactions which are impossible in aqueous media due to kinetic or thermodynamic constraints become possible in non-aqueous solvents. The recent discovery that biocatalysts can function in extreme environments such as organic solvents has important implications for the implementation of biosensor technology in formerly inaccessible environments. It also extends the number of detectable analytes to include poorly water soluble organic species in the petrochemical, food and environmental areas. Chapter 4 deals with the electrochemistry of the anticancer drug 7-OH-coumarin and the development of an immunosensor for this compound. The final experimental chapter deals with the detection of some metal ions using a plant tissue modified carbon paste electrode, and includes speciation studies of copper.