Cytotoxicity does not define a specific cellular death mechanism (apoptosis or necrosis), but is the cell-killing property of a chemical compound independent from the mechanisms of death. Most current assays for measuring cytotoxicity in vitro are based on alterations of plasma membrane permeability and the consequent release (leakage) of components into the supernatant or the uptake of dyes, normally excluded by viable cells. In this paper, the most often used methods for studying cytotoxicity in vitro are described.