This lecture series is focused on the fate of xenobiotic compounds, defined as being components of our chemical environment which are foreign to the metabolic network of an organism. The relation between chemical structure, biotransformation, and toxic, in particular genotoxic, properties of xenobiotics will be studied. The enzyme complexes that are of particular importance for bio-activation and bio-deactivation of xenobiotics (cytochrome P-450 oxidase, epoxide hydrolase, glutatione transferase, sulfo transferase, esterases, etc.) will be discussed in detail. Case studies: Special attention will be given to the phenomenon of bio-activation at the molecular level of a number of important classes of potentially toxic compounds, such as halogenated hydrocarbons (solvents), aromatic amines, aflatoxins, polycyclic aromatic hydrocarbons (PAH’s), pyrolysis products (frying, roasting), and chlorinated dibenzodioxins and dibenzofurans.