Cell viability is a measurement of how healthy a cell is. However, the definition of cell viability is dependent on the researcher and their experimental goals. A few ways one can measure cell viability includes looking at apoptosis, cell cycle, or even metabolism. There are a number of assays to assess various parameters and a number of methods (i.e. flow cytometry or immunocytochemistry). Regardless, of how you choose to measure cell viability, there is an assay to help you do so.