Cytochrome P450 (CYP) enzymes catalyze the generation of reactive species capable of binding with cellular macromolecules, leading to acute and delayed toxicity. Since individual CYP forms differ markedly in their substrate preferences and regulation, the expression profiles of CYP in various cell types are important determinants in tissue-specific toxicity. The highest concentrations of most forms of CYP are found in liver, but they are also present in many extrahepatic organs. Liver is also a target organ in which CYP-mediated activation and toxic outcome have been most convincingly linked. Prime examples are paracetamol-induced hepatotoxicity and aflatoxin B1-associated hepatic cancer. In contrast to liver, most extrahepatic tissues are composed of multiple call types, which make experimental approaches difficult. Also the low abundance of individual forms is a challenge in the study of extrahepatic CYP-related toxicity. Recent years have witnessed the emergence of molecular biological techniques, e.g., reverse transcriptase-polymerase chain reactions, which facilitate the study of low abundant CYP forms in human tissues. Nevertheless, in the end we need definite information on the expression of activity, and for this purpose enzyme-specific substrates, reactions, and inhibitors and other methods to detect proteins and associated activities are needed. In humans, it is important to measure activities of specific enzymes in vivo. For this purpose, two approaches are currently available. Metabolism and/or elimination of enzyme-specific drugs can be employed. In cases in which genetic background determines the presence or absence of a specific enzyme, phenotyping and genotyping tests can be devised, e.g., for CYP2D6 (debrisoquine hydroxylation) polymorphism.