Enzyme sensors for diagnostic and environmental applications have developed by immobilizing biocatalysts onto the active solid surface of physico-chemical sensors. The performances of a biosensor are mainly related to the properties of the bioreceptor material and the techniques used for its immobilization onto the transducer. Different enzyme immobilization techniques will be presented: immersion, direct binding, reinforced membranes and finally aerosol vaporisation that combines adsorption and covalent binding of proteins to obtain ultrathin bioactive films.