This chapter analyzes the results of the immobilizations, to introduce some unpublished data on the methods available for immobilization of luciferase, and to discuss the potential of these methods. Immobilized iuciferase preparations generally have a longer transient period before maximum light emission is achieved. The immobilized luciferase produced from the doubly activated membrane showed a small increase in activity but this was thought to be too low relative to the increased effort needed to obtain it. A much more sensitive process involves the luminescence reaction catalyzed by firefly luciferase. This can be used for the analysis of Adenosine triphosphate (ATP) down to femtomolar concentrations and with a linear response up to micromolar concentrations. Many methods suggested for the analysis of ATP. Such methods have received close attention from microbiologists as the concentration of viable microorganisms is closely related to that of the ATP they contain and the endogenous ATP may be released by simply lysing the cells using various reagents.