Mycorrhiza is a mutualistic association between fungi and the roots of the vast majority of terrestrial plants. In natural ecosystems, the plant nutrient uptake from the soil takes place via the extraradical mycelia of these mycosimbionts. While most herbaceous plants and tropical trees form endomycorrhiza-type interactions, trees of boreal and temperate ecosystems are typically ectomycorrhizal (ECM). These species include the majority of ecologically and economically important trees and the fungal symbionts are predominantly filamentous basidiomycetes. The symbiotic phase in the life cycle of ECM basidiomycetes is the dikaryon. This book reviews studies on symbiotic relevant gene functions.