Orchids belong to one of the largest families of flowering plants and their survival in nature is intrinsically bound to the symbiotic relationship with mycorrhizal fungi. The study of orchid mycorrhiza is particularly complex because of the peculiar life cycle and diverse trophic strategies of these plants. We have been investigating several aspects of orchid mycorrhizal biology, from diversity and distribution of fungal symbionts to the molecular bases of plant-fungus interactions, in juvenile as well as adult stages of terrestrial orchid species. Despite the peculiarities of this cheating symbiosis, many molecular markers are shared between orchid mycorrhiza and other endosymbiotic interactions.