The medical use of herbs is widespread. In a multi-ethnic group of patients attending an emergency department in New York 22% reported that they used herbal medicines; use was highest among Asians, 37%. It is important therefore for clinicians to document use of herbal medicines as part of the patient's drug profile. Plants synthesize complex (organic) molecules for their structure and function, and therefore are a rich source of chemicals. Active chemicals purified from plants are accepted effective medications e.g. digoxin, and morphine. When parts of plants or crude extracts of plants are used for medicinal purposes they are called herbal medicines. What is the active ingredient in herbal medicines? Most herbal medicines contain hundreds of chemicals and there is often no agreement as to which is potentially active. An example is garlic, comprised of many sulfur compounds, several of which are candidates for possible therapeutic activity. How can one be sure that herbal medicines contain what is on the label? The only regulatory requirement in The US markets is that all products intended for medicinal use, including natural health products, are issued a Drug Identification Number. However, these numbers are not required for raw materials such as bulk herbs. Herbal medicines are not required to pass any regulatory analysis to be sold as a health food supplement. There are many examples of herbal medicines that have been adulterated with other (more toxic) herbs, potent drugs (e.g. phenylbutazone, synthetic corticosteroids and other prescription drugs), or heavy metals. How are herbal medicines standardized? The chemical constituents of plants vary depending on the species, variety and part of the plant, with conditions of growth (soil, water and temperature), and with the age of the plant. These complexities and variations of chemical content make standardization essential. In some cases standardization is attempted, but it is difficult and seldom accomplished. Plants are rich sources of chemicals and potential sources of effective herbal medicines. However, more research, regulation, and standardization are required before herbal medicines can be recommended as effective and safe therapies.