The examples given in this presentation show that the levels of secondary metabolites can be modified by genetic engineering methods. In these methods, the specific genes encoding enzymes involved in key steps of biosynthetic pathway can be transferred and expressed in cell lines, hairy root cultures or transgenic plants of the same or another species. Overexpression of the enzymes may lead to the accumulation of intermediates of pathways and increased synthesis of desirable products. Best results will be obtained when the overproduced enzyme activity is clearly the rate-limiting step in the pathway. Therefore, better knowledge of the limiting steps of the pathways is required. The successful of biosynthetic pathways regulation may also be achieved by expressing antisense RNA which is complementary to the mRNA encoding pathway enzyme or by genetic manipulation of degradative pathways. It should be mentioned that genetic engineering also plays a role in the development of novel plant-derived drugs. As secondary metabolites are important in the defense of plants against pathogens, such engineered plants may show an increase in resistance against pathogens.