Herbal treatments are becoming increasingly popular, and are often used for internal as well as dermatological conditions, both externally as well as orally. The prevalence of contact sensitization against several plants especially of the Compositae family is quite high in Europe. Sensitization seems to occur relatively frequent with a few species such as arnica, elecampane and tea tree (oil), and occurs rarely with the majority. Testing for plant allergy is problematic because of the limited number of commercially available standardized patch test substances and the danger of active sensitization when testing with plants, parts thereof, or individual extracts. Knowledge about the allergic potential of plants is limited. Although plants are regarded as critical allergens by dermatologists, the number of reported cases of contact dermatitis is relatively small. Many widely used substances are not licensed as drugs or cosmetics. While the positive effects are frequently questionable or limited, the side effects are often more evident. Adverse effects of herbal medicines are an important albeit neglected subject in dermatology, which deserves further systematic investigation.