Virtually all cultures have, throughout history, used a variety of plants or materials derived from plants for the prevention and treatment of disease. Evidence of the beneficial therapeutic effects of these medicinal herbs is seen in their continued use. Additionally, the development of modern chemistry permitted the isolation of chemicals from medicinal herbs that have served as drugs or starting materials for the synthesis of many important drugs used today. Many more modern drugs have been synthesized as a result of knowledge gained from studies of mechanisms of actions of chemicals first isolated from medicinal herbs. Thus, medicinal herbs have played a major role in the development of modern medicine and continue to be widely used in their original form. Whereas it is generally agreed that most medicinal herbs are safe under the conditions used, some are toxic and should be avoided even though they are readily available, and others have significant adverse side effects when misused. Also, little has been done to investigate potential adverse effects that may be associated with extended or high-dose use of medicinal herbs. Thus, concern has been expressed that the lack of quality control used in the preparation of medicinal herbs, plus their unregulated sale and uninformed use, pose potential adverse health effects for consumers. There is also concern regarding potential herb/herb or herb/drug interactions and possible untoward health effects of medicinal herbs in sensitive subpopulations such as the young and the elderly and certain genetically predisposed individuals. In this paper, we discuss these concerns at some length and make recommendations for additional research and education discussed in the recent International Workshop to Evaluate Research Needs on the Use and Safety of Medicinal Herbs.