Control of Microbiological contamination in cooling towers requires an efficient means of measuring the contamination.  ATP-based testing methods offer immediate results and measure the total microbial population, both aerobic and anaerobic, in one step.  Methods for spot-checking towers, measuring biocide efficacy, detecting biofilms, and monitoring biofilm clean-up are described.  Differential effects of oxidizing and non-oxidizing biocides on ATP response curves are demonstrated in specific examples.