Ginseng is one of the most popular herbal dietary supplements in the U.S. market, with five to six million persons using it even before the recent boom in the herbal supplement industry. Ginsana (G115 ginseng), a standardized extract of Panax ginseng (Asian ginseng, also called Chinese or Korean ginseng) controls half of this market. Numerous reports of adverse effects from products containing ginseng have been filed with the U.S. Food and Drug Administration (FDA). The literature also documents “ginseng abuse syndrome” among regular users. The chronic effects of ginseng are not well characterized. Because of its significant human exposure and little information on toxicity is available, Panax ginseng has been nominated by the U.S. National Institutes of Health (NIH) to the U.S. National Toxicology Program (NTP) for assessing its carcinogenic potential. In this review, we discuss the environmental occurrence, purported therapeutic effects, biological effects, and toxicity of ginseng, with focus on Panax ginseng. To demonstrate how NTP chronic tumorigenicity bioassays are conducted, the tumorigenicity bioassay of Panax ginseng is detailed described.