Herbal medicines are frequently used in combination with conventional drugs and interactions are likely to be more common than those which manifest clinically. Pharmacokinetic interactions mediated by drug-metabolizing enzymes or transporters are involved in many herb-drug interactions. Polymorphisms in the genes for these enzymes and transporters may influence the interactions mediated through these pathways. Herb-drug interactions can be identified in vivo using the herbs with individual probe drugs or drugs with a narrow therapeutic index. A probe drug cocktail approach offers a more efficient screening procedure. If potential interactions are studied in subject groups with different genotypes, more definitive information can be obtained. The selection of genotype groups to study should depend not only on the major enzymes or transporters involved in the disposition of a drug, but also on alternate pathways which may become more important in subjects with reduced activity in the primary pathway, and the pathways for pharmacodynamic effects should also be considered for some drugs. Some genotypes may only be found in certain ethnic groups. Currently, the pharmacogenetic approach has been used mainly to study drug-drug interactions so there is considerable potential to extend this to the study of herb-drug interactions in vivo.