An elementary view of basic toxicity mechanisms will be offered. The unit is structured at several different levels. First, the basic toxicity mechanisms in the molecular field will be introduced. This will explain what requirements are necessary for the interactions between target toxicity molecules and xenobiotics, what types of reactions are possible, etc. The second level of this teaching unit is the mechanisms of cellular toxicity (cytotoxicity). That is, the metabolic changes that the cell must undergo to trigger the mechanisms of cell death will be explained, either by the route of necrosis or by the route of apoptosis.Finally, some of the most common toxicity phenomena in the field will be explained tissue. Chemical carcinogenesis and teratogenesis and developmental alterations have been chosen as examples because they are the most representative of the toxic effects induced by xenobiotics.