David Don was the first to recognize this group as a tribe within the Ericaceae, later raised to subfamily status as the Monotropoideae by Asa Gray in 1878. (However, George Arnott Walker-Arnott was the first to validly publish that name, as a subfamily of Monotropaceae, in 1832, hence, Arnott is cited as author of the name.) Other classifications have included Monotropoideae as a subfamily of Hypopityaceae, by August W. Eichler (1875), and as a subfamily of Pyrolaceae by Carl Georg Oscar Drude (1889); both classifications united the monotropes with the pyrolids in a single group. Over the next century, authors have variously treated this group as a distinct family or as a subfamily of the Ericaceae, though the trend from Margaret W. Henderson (1919) onward was toward the latter subfamily classification, albeit, the influential Cronquist and Dahlgren systems continued to treat the group as the family Monotropaceae, separate from the Ericaceae. Contemporary molecular phylogenetics has clearly established the Monotropoideae as a group within the larger Ericaceae, though many of the details of relationships between the Monotropoideae and the rest of the Ericaceae are still (as of 2015) a topic of active research, particularly the question of whether or not the Pyroleae and the rest of the Monotropeae form a single monophyletic group. The species in this subfamily are all mycoheterotrophic, relying on fungal hosts for their carbon nutrition. The fungi parasitized by these plants are ectomycorrhizal species of fungi. Hence, these plants act as direct parasites of these fungi, and also indirectly, act as an epiparasite of conifers and the larger shared mycorrhizal network. Monotropoideae species can generally be described as full, obligate mycoheterotrophs, though if the Pyroleae are treated as part of the Monotropoideae, include partially mycoheterotrophic (mixtotrophic) members as well. The parasitism by these plants is generally very specific in terms of its fungal hosts, ranging from single families of fungi, to a few closely related species. The morphology of the root and the root-level fungal symbiont is distinctive and referred to as monotropoid mycorrhiza.