Synergistic interactions are of vital importance in phytomedicines, to explain difficulties in always isolating a single active ingredient, and explain the efficacy of apparently low doses of active constituents in a herbal product. This concept, that a whole or partially purified extract of a plant offers advantages over a single isolated ingredient, also underpins the philosophy of herbal medicine. Evidence to support the occurrence of synergy in within phytomedicines is now accumulating and is reviewed here. Synergistic interactions are documented for constituents within a total extract of a single herb, as well as between different herbs in a formulation. Positive and negative aspects of interactions are discussed together with the methods used to identify and measure synergy. The evidence is divided into experimental, in vitro instances, as well as clinical examples where available. Herbs discussed include Ginkgo biloba, Piper methysticum (Kava-Kava), Glycyrrhiza glabra, Hypericum perforatum, Valeriana officinalis, Cannabis sativa, Salix alba and others.