Secondary metabolites from plants serve as defense against herbivores, microbes, viruses, or competing plants. Many medicinal plants have pharmacological activities and may, thus, be a source for novel treatment strategies. During the past 10 years, we have systematically analyzed medicinal plants used in traditional Chinese medicine and focused our interest on Artemisia annua L. (qinhao, sweet wormwood). We found that the active principle of Artemisia annua L., artemisinin, exerts not only antimalarial activity but also profound cytotoxicity against tumor cells. The inhibitory activity of artemisinin and its derivatives towards cancer cells is in the nano- to micromolar range. Candidate genes that may contribute to the sensitivity and resistance of tumor cells to artemisinins were identified by pharmacogenomic and molecular pharmacological approaches. Target validation was performed using cell lines transfected with candidate genes or corresponding knockout cells. The identified genes are from classes with diverse biological functions; for example, regulation of proliferation (BUB3, cyclins, CDC25A), angiogenesis (vascular endothelial growth factor and its receptor, matrix metalloproteinase-9, angiostatin, thrombospondin-1) or apoptosis (BCL-2, BAX, NF-kappaB). Artesunate triggers apoptosis both by p53-dependent and -independent pathways. Antioxidant stress genes (thioredoxin, catalase, gamma-glutamylcysteine synthetase, glutathione S-transferases) as well as the epidermal growth factor receptor confer resistance to artesunate. Cell lines overexpressing genes that confer resistance to established antitumor drugs (MDR1, MRP1, BCRP, dihydrofolate reductase, ribonucleotide reductase) were not cross-resistant to artesunate, indicating that artesunate is not involved in multidrug resistance. The anticancer activity of artesunate has also been shown in human xenograft tumors in mice. First encouraging experience in the clinical treatment of patients suffering from uveal melanoma calls for comprehensive clinical trials with artesunate for cancer treatment in the near future.