ATP is a molecule found in and around living cells, and as such it gives a direct measure of biological concentration and health. ATP is quantified by measuring the light produced through its reaction with the naturally occurring firefly enzyme luciferase using a luminometer. The amount of light produced is directly proportional to the amount of ATP present in the sample.  
ATP tests can be used to: 
* Control biological treatment reactors
* Guide biocide dosing programs
* Determine drinking water cleanliness
* Manage fermentation processes
* Assess soil activity
* Determine corrosion / deposit process type
* Measure equipment or product sanitation