Toxicological research utilizes various approaches and methodologies to identify hazards and to understand the basic processes which influence the expression of toxicological responses in biological systems. A particularly fruitful approach has been to identify and develop in vitro model systems which retain the basic characteristics of the more complex in vivo condition, yet can be experimentally manipulated for research purposes. In vitro research models have several inherent properties which give them significant advantages in many areas of toxicological research. First and foremost, in vitro systems allow for control over experimental conditions which are not possible in vivo. Physical and chemical conditions of the cellular environment, such as temperature, pH, pO2 and concentration of physiological ions, can be rigorously controlled over wide ranges, including non-physiological conditions, to elucidate fundamental mechanisms. Biological factors which influence cellular responses, such as hormones and mediators, can be studied individually and in combinations which would not be possible in vivo.