Firstly described by Linné (1753, Vol.1, page 387-388) as Monotropa hypopitys and accepted as such by Wallace (1995). However, molecular phylogenies have revealed that Monotropa uniflora is more closely related to Monotropastrum humile, whereas Monotropa hypopitys seem to be sister of Pityopus californicus (Bidartondo and Bruns 2001, Tsukaya et al. 2008). Therefore, some standard taxonomies (Stevens et al. 2004, Seybold 2011) now use Hypopitys monotropa coined by Crantz (1766), which we used to follow here for long. However, the standard naming sources of the 2020th, POWO as well as WFO, treat the name Hypopitys monotropa as well as its subspecies as synonyms of Monotropa hypopitys.
According to Wood (1961, footnote on page 68), the generic name Hypopitys by Hill (1756) is illegitimate, since it has been explicitly ("This author (Linné) takes away its received name hypopitys, and calls it monotropa") coined as a substitute for Linné's Monotropa due to earlier names. In contrast, Crantz' (1766) much shorter description but without the verbal connection to Linné and keeping the generic name as epitheton, is legitimate. The spelling of the genus or epitheton has changed several times (Hyopithys, Hypopithis). Wallace (1987) prefers the spelling 'hypopitys' (after pitys (gr.) = spruce, pine, fir), probably due to Crantz' spelling in 1766. Nevertheless, Linné (1753) and even Wallace (1975) himself spelled it 'hypopithys' before.
The synonymy list is long (84 synonyms!). Check Wallace (1975), who also do not accept the widely used two subspecies M. hypopitys ssp hypopitys and M. hypopitys ssp hypophegea. M. hypopitys is widely distributed throughout North- and Central America, Europe and Asia with gaps in the dryer parts of these continents. Bidartondo and Bruns (2001), infact, detected a Swedish clade, an Eurasian clade and a north American clade within this species, which might point to the existence of cryptic species.