Many orchids require mycorrhizal symbioses with fungi for their development and survival. Rhizanthella gardneri the Western Australian underground orchid is associated with the companion plant Melaleuca uncinata and its ectomycorrhizal fungus symbiont. Much less is known about the habitat requirements of its sister species, R. slateri, which occurs in Eastern Australia. The absence of chlorophyll from Rhizanthella gardneri and R. slateri results in total dependency on associations with fungal symbionts. Many ecological and biological aspects of these fascinating orchids remained poorly known, including the identity of the fungal associates and the nature of their tripartite associations with Rhizanthella and Melaleuca. Extremely high specificity of these mycorrhizal relationships is likely to be the most important factor explaining the highly specific habitat requirements of underground orchids. The purpose of this study was to conduct further investigations of the role of the mycorrhizal associations of Australian underground orchids by identifying the fungi involved in these associations, optimising their growth in sterile culture and devising efficient means for synthesising their tripartite associations with R. gardneri and M. uncinata. In total, 16 isolates of fungi were successfully obtained from the two underground orchids and used in a series of experiments to understand both the nature of the fungi and their relationship with orchids. The identity of these fungi was established by using conventional morphological and molecular methods. Cultural and morphological studies revealed that all isolates from R. gardneri and R. slateri were binucleate rhizoctonias with affinities to members of the genus Ceratobasidium. However, the teleomorph state that was observed from the R. slateri symbiont during this study more closely resembled a Thanatephorus species. Further identification using ITS sequence comparisons confirmed that mycorrhizal fungi of Rhizanthella belonged to the Rhizoctonia alliance with relatives that include Thanatephorus, Ceratobasidium, or Rhizoctonia from other continents with over 90% similarity. Most of these related fungi are known as plant pathogens, but some were orchid mycorrhizal fungi. However, the isolates from the two underground orchids were most closely related to each other and formed a discrete group relative to other known members of the Rhizoctonia alliance. Sterile culture experiments determined culture media preferences for mycorrhizal fungi from Rhizanthella and other orchids. A fully defined sterile culture medium designed to more closely resemble Australian soil conditions was formulated. This new medium was compared to undefined media containing oats or yeast extract and recommendations for growth of these fungi are provided. The undefined media based on oats provided the best growth of most fungi, but the new Australian soil media was also effective at growing most orchid mycorrhizal fungi and this fully defined media was less prone to contamination and should provide more reproducible results. A comparison of three methods for inoculating M. uncinata with the underground orchid fungi resulted in the production and characterisation of ectomycorrhizal roots and hyphae formed by fungi isolated from R. gardneri and R. slateri. These underground orchid fungi could easily be distinguished from other mycorrhizal fungi (caused by airborne contamination) by the characteristic appearance of these roots and hyphae. A new system for growing and observing tripartite mycorrhizal associations was devised using pots with side viewing windows and the use of transparent seed packets to contain Rhizanthella seeds. This method allowed all the stages of seed germination to be observed in the glasshouse, culminating in the production of underground orchid rhizomes. Seed germination was only successful when seed was placed directly over active M. uncinata ectomycorrhizas confirmed to belong to the correct fungus by microscopic observations through the side of window pots. The importance of these new scientific discoveries concerning the biology and ecology of the underground orchids and their associated fungi for the recovery of these critically endangered orchids are discussed.