There are several situations in which conventional crosslinking-based immobilization is inadequate in the construction of microelectrodes, for example, when on-wafer deposition (i.e., immobilization on the whole wafer before it is diced up into individual devices) is required, leading to many localized immobilizations, or during the fabrication of multianalyte sensors needing several distinct enzyme membranes. The three main types of immobilization developed to overcome these problems are based on photochemistry, electrochemistry, or printing. In this chapter we shall deal with the first, photochemical-based immobilization. Two main types of photochemical immobilization have emerged, chemical crosslinking followed by lift-off (1 –3 ) and entrapment in photopolymerized gels (4 –9 ). We will illustrate both approaches with the immobilization of glucose oxidase on a silicon wafer.