The body has mechanisms that remove unwanted molecules and convert them into derivaties of the administered molecule. These derivaties are more prone to elimination, usually by the liver. This process of “metabolism” usually leads to the formation of molecules that are more water soluble. The metabolites of a drug can, however, also possess pharmacolgical and / or toxicological activity, which may be benefical or detrimental to the system. It is vital, therefore, to be able to detect, identify and investigate the drug metabolites so that the effects of the administered agent can be monitored accurately. Using in vitro techniques, the fate of drugs in biological systems can be followed and the reaction mechanism(s) can be elucidated. Understanding, the pathways of metabolism enables the pharmacists and doctors to understand and control the use of drugs more effecively and safely. In the present paper, the techniques studying the in vitro oxidate microsomal phase I metabolism of drug molecules will be reviewed. Keywords :