In the scope of the present work the effects of an arbuscular mycorrhiza on plant yield, on the quality of plant ingredients and on the plant state of health of herbal and medicinal plants have been analysed, evaluated and discussed using the example of marjoram, thyme and St. John’s wort. Mycorrhiza-dependent yield increases have especially been determined for thyme. Also increases of essential oil yields were most evident for this culture. In some cases mycorrhization also yielded in an enhancement of the thyme-typical ingredients p-cymene and thymol. Mycorrhization has also been shown to have positive effects on hypericin amounts in St. John’s wort, what is important for its use as phytopharmaceutical. Most important influences of mycorrhization on yield and plant ingredients have especially been observed during unfavourable climatic conditions. Furthermore it has been shown during the investigations, that a well established mycorrhiza can improve the tolerance of St. John’s wort plants to the wilt disease inducing pathogen Colletotrichum cf. gloeosporioides, so that plants can be saved from stronger damage. In mycorrhizal plants antioxidative enzymes have been stabilised over a longer period of time, antioxidants have been regenerated and also the development of lipidperoxidation has been inhibited in mycorrhizal plants compared to non-mycorrhizal plants. Finally, this paper presents for the first time also a systemic influence of mycorrhization on the protein expression in the overground plant material of the model plants marjoram and St. John’s wort. Therefore a two-dimensional electrophoresis method for the separation of proteins of the aerial plant material of adult marjoram and St. John’s wort plants was successfully established and optimised. Especially photosynthesis-related proteins and stress-related enzymes have been differentially regulated in response to mycorrhization.