Ericoid mycorrhizal (ERM) fungi are a diverse assemblage of symbiotic fungi that features culturable ascomycetes in the Helotiales and Onygenales, but also so far unculturable basidiomycetes in the Sebacinales. They form a distinct endomycorrhizal association with some plant genera in the Ericaceae. ERM plants dominate in heathlands characterised by very poor nutrient status and considerable edaphic stress, and their success in these harsh environments is ascribed to the functional traits of their symbiotic fungi. ERM fungi are able to exploit recalcitrant organic substrates thanks to an arsenal of extracellular enzymes. They also display adaptive mechanisms of stress tolerance and are able to withstand high concentrations of toxic compounds such as heavy metals. ERM plants are also commonly found as understorey vegetation in woodland habitats, and molecular investigations on the genetic diversity of ERM fungi, together with cross-inoculation experiments under gnotobiotic conditions, indicate the potential networking ability of these fungi in mixed plant communities.