It is clear that quinone-thioethers possess a variety of biological and toxicological activity [5]. The ubiquitous nature of quinones and the high concentrations of GSH within cells virtually guarantees that humans will be exposed to the potential adverse effects of the resulting quinone-thioethers. The generation of a biologically reactive intermediate is usually the initial and necessary step that eventually results in cell death, tissue necrosis, and/or tumor formation. The various mechanisms in which reactive intermediates interact with cellular constituents and trigger events that lead to cell death or cell transformation, are only now becoming unravelled. Knowledge of the disposition of quinone-thioethers will therefore be an important prerequisite to understanding their mechanism of action. Studies on the occurrence and biological and toxicological activity of quinone-thioethers therefore will be an important area for future research.