The use of herbal medicine is widespread and growing, with as many as 3 in 10 Americans using botanical remedies in a given year. Because many herbal medicines have significant pharmacological activity, and thus potential adverse effects and drug interactions, healthcare professionals must be familiar with this therapeutic modality. This article summarizes the history and current use of plant-based medicine and highlights the evidence of the risks and benefits associated with 6 plants: echinacea, garlic, ginger, ginkgo, St John's wort, and valerian. Therapies outside the medical mainstream tend to suffer from a dearth of research and critical evaluation. Critics and supporters alike note the conceptual and practical difficulties in studying many complementary and alternative therapies such as acupuncture, homeopathy, and meditation. Herbal medicine, however, lends itself well to standard evaluation methods. This article summarizes and evaluates evidence from randomized controlled trials and meta-analyses. We present the results of meta-analyses and subsequent randomized controlled trials for garlic and St John's wort; a comprehensive critical review and subsequent randomized controlled trials for ginkgo; and summaries of all identified randomized controlled trials for echinacea, ginger, and valerian.