Used to measure cell viability and proliferation by detecting ATP levels in cells. Living cells produce ATP, so the amount of ATP present correlates directly with the number of viable cells. The assay works by adding a lysis buffer and luciferase enzyme system that reacts with ATP to produce light. The light output is measured using a luminometer or microplate reader, providing a rapid, sensitive, and quantitative assessment of cell health. Commonly used in drug screening, cytotoxicity testing, and cell growth studies. Requires minimal handling and delivers results faster than traditional methods like MTT assays. Suitable for high-throughput applications in both adherent and suspension cell cultures.