Instrumentation for the measurement of bioluminescence and chemiluminescence have been subject of many publications. This chapter examines "luminometer" for this type of instrument, although other names, like radiometers or photometers, are used occasionally. The use of noncommercial luminometers is the exception. These luminometers are characterized by features like user friendliness, increased reliability, and a higher degree of automation inlcuding sample preparation, automatic quality control of instrumentation and reagents, and advanced software reporting final results. When associating bio and chemiluminescence to microbiology, one traditionally thinks of bioluminescent adenosine triphosphate (ATP) techniques. The assay technologies hold the promise to add specificity to sensitivity in rapid microbiology. For ATP bioluminescence, however, the quantum efficiency for bialkali type photomultipliers is reduced to about 3 to 6%, according to the tube model and individual efficiency variations. ATP/firefly luminescence is frequently claimed to provide a virtually constant light signal with quality commercial reagents. In ATP measurements, bacterial contamination of the injection system lead to incorrect results.