The most important enzymes of detoxication are cytochromes P450 of Phase I and UDP-glucuronosyltransferases of Phase II of drug metabolism. The conventional division of drug, or xenobiotic, metabolism to two phases has survived almost fifty years and although not perfect, still is rather informative and practical. The recent addition of the next, third phase to the first two (Phase I as yielding a molecule with free functional groups ready for conjugation with another molecule or its part in Phase II) stressed the importance of drug transport across the membranes even if it is not a pure metabolic process (i.e. a process changing the polarity and structure of a compound). The importance of testing the toxicity of newly invented drugs and drug candidates is also given by the simple fact that it is the toxicity and unwanted pharmacokinetics incl. interactions which is the reason for declination of one half of new or promising compounds. All other reasons as low efficacy, adverse effects, marketing and commercial reasons, and other reasons are responsible for the second half of the rejections (Van de Waterbeemd & Gifford, 2003). According to the recent view of the regulation authorities, the in vitro testing of drug metabolism of any new chemical entity should be realized together with an evaluation of its interaction with the system of drug transporters, mainly with the ABC membrane based protein pumps which often decide on the drug bioavailability. The majority of known cases of drug toxicity are however ascribed to reactions in which the catalytic processes involving cytochromes P450 are involved. The death toll caused by malignant arrhythmias due to increased level of terfenadine or astemizole is well documented and is a part of each modern textbook of pharmacology and toxicology. Also, the recent case of mibefradil which has been withdrawn from the market by the company two years after successful introduction is also well known. In all cases, increased levels of the drug were caused by interactions with other drugs, competing for the same form of cytochrome P450. Cytochromes P450 are also known for their two-sided behavior often compared with Dr. Jekyll and Mr. Hyde of the R. L. Stevenson′s novel: The same form is responsible for detoxication processess as well as by activation of a carcinogen or a toxicant, in principle, by the same type of chemical reaction, i.e. by activation of molecular oxygen. It is of course not the fault of the cytochromes P450, but our own – as the enzyme is simply performing the same role independently on our wish. The ways and approaches used for in vitro studies of processes in which cytochromes P450 take place can be divided according to the degree of organization of the system studied. From the most simple to the most complex, the first system should comprise only the molecules of the enzyme with the necessary components assuring the proper assembly and function.