We present an analysis of fungal specificity in myco-heterotrophic orchids and monotropes. We argue that specificity represents a continuum and can only be properly assessed using phylogenetic data. Several green orchids associate with wide phylogenetic arrays of Rhizoctonia species, and hence show little specificity, while other green orchids, and all studied achlorophyllous orchids and monotropes, associate with narrow phylogenetic groups of fungi, and hence show significant specificity. In several species, this tight specificity has been shown to apply from seed germination through adulthood under natural conditions, though not necessarily under in vitro conditions. Patterns of specificity have been correlated with patterns of fungal distribution and habitat variation in several myco-heterotrophs. However, studies of other myco-heterotrophs have shown that tight specificity is expressed even when diverse fungi co-exist with the plant. Moreover, in one case, genetic influences of the host plant have been shown to outweigh environmental influences over the patterns of specificity. Major host jumps and intraspecific host-race formation have contributed to the evolution of specialisation in several myco-heterotrophs. Some achlorophyllous orchids associate with wood-decay or parasitic fungi, but many recent studies have revealed associations with ecto-mycorrhizal fungi in orchids, monotropes, and a liverwort. Tracer studies show that autotrophic ecto-mycorrhizal host plants can provide the fixed carbon to nourish myco-heterotrophs linked by a shared fungal partner. Important outstanding questions concern recognition phenomena, the origins and evolution of specificity, the physiology and ecology of carbon exchange, and whether myco-heterotrophs interact with fungi in fundamentally different ways than do autotrophs.