Herbal medicines are widely used in the United States, with approximately one quarter of adults reporting use of an herb to treat a medical illness within the past year. Herbs contain complicated mixtures of organic chemicals, the levels of which may vary substantially depending upon many factors related to the growth, production, and processing of the herbal product. While many manufacturers attempt to provide products with consistent levels of suspected active ingredients through a process known as standardization, this technique has uncertain effects on the safety and efficacy of the final product. Herbs are considered to be dietary supplements in the United States and therefore are subjected to a very limited form of regulation and oversight. Although herbs are often believed to be “natural” and therefore safe, many dangerous and lethal side effects have recently been reported, including direct toxic effects, allergic reactions, effects from contaminants, and interactions with drugs and other herbs. Of the ten most commonly used herbs in the United States, systematic reviews have concluded that only four are likely to be effective, and there is very limited evidence to evaluate the efficacy of the approximately 20,000 other available herbal products. Because herbs may contain potent bioactive substances and are often marketed to treat specific diseases, many have argued that they should be subject to more stringent regulation, similar to over-the-counter drugs. To improve the safety and consistency of herbs, additional research is needed to define the pharmacology, stability, and bioavailability of these products.