Bioluminescent measurement of ATP using luciferin/luciferase system is a very sensitive, specific, rapid, and convenient method in microbiology. The use of this method to quantitate viable microbial cells requires the use of various validation procedures and safeguards, depending on the specific application. Most industrial applications of bioluminescent ATP measurement involve detection and quantitation of microbial contamination in a particular product, such as the rapid identification of a critical level of contamination in output material. Cloning of the firefly luciferase gene lux promises to be a much more efficient, consistent and inexpensive source of recombinat enzyme. With the realization of this potential and the marketing of automated luminometers that will undoubtedly be improved greatly in terms of throughput capacity, bioluminescent ATP measurement is becoming the method of choise for large-scale monitoring of viable microbial biomass.