Orchid mycorrhizas (OM) are symbiotic interactions between fungi and terrestrial, epiphytic or lithophytic species of the Orchidaceae. In the association, fungal hyphae enter parenchyma cells of germinating seeds, protocorms, seedlings or roots of adult plants, and form elaborate intracellular hyphal coils. The latter are known as pelotons, thought to be the site of nutrient transfer between the symbionts, which is essential for the perpetuation of orchids in their natural habitats. OM represent a quite obscure plant-microbe interaction. They are found in one of the most species-rich plant families on earth, since understanding of their biology may assist in successful conservation efforts on threatened orchid species and as they actually contribute to the health of forest, woodland and grassland ecosystems. In addition, some orchid mycorrhizal systems can be easily manipulated in vitro, making them a useful model to investigate the molecular physiology of mycorrhizal associations specifically, and to make comparisons with other plant-microbe interactions generally.