The phenomenon “Mycorrhiza” comprises all symbiotic associations of soil-borne fungi with roots or rhizoids of higher plants and was first introduced into the literature by Frank (1). The group of fungi and plants, which are involved in the interaction, determines the type of mycorrhiza they form (2). Arbutoid and monotropoid ectendomycorrhizas, as well as the ericoid and orchidoid endomycorrhizas, can only be found in a few plant species and are, therefore, restricted to certain ecosystems. In contrast, ectomycorrhizas are established between a great variety of mostly Basidiomycota and the roots of many woody plants. This symbiosis is very common in forest ecosystems in the temperate zone and can be applied to plant production systems in tree nurseries (3). The most widespread type, however, is represented by the arbuscular endomycorrhiza, and this chapter concentrates on this type of symbioses, because the plant hosts include the most important crops used in agri-and horticulture.