The rhizome is usually thin and wiry and the fronds variously pinnate with a single strand ("nerve") of vascular tissue.

The cuticle is also greatly reduced or absent,[3] leaving filmy ferns poikilohydric and very susceptible to desiccation where a reliable water supply is not present.

In the molecular phylogenetic classification of Smith et al. in 2006, the Hymenophyllales, containing the single family Hymenophyllaceae, were placed in class Polypodiopsida sensu stricto (the leptosporangiate ferns).

Recent molecular phylogenic studies do show two distinct monophyletic clades of fairly equal size, but they are only roughly aligned with the two traditional genera.

[10] The genera used in PPG I and the subgenera assigned by the system of Ebihara et al. are: Hymenophyllum Abrodictyum Cephalomanes Trichomanes s.s. Callistopteris Polyphlebium Didymoglossum Crepidomanes Vandenboschia Hymenophyllum Callistopteris Cephalomanes Abrodictyum Trichomanes s.s. Polyphlebium Didymoglossum Vandenboschia Crepidomanes The great majority of the species are found in tropical rainforests, but some also occur in temperate rainforests (particularly New Zealand, with 25 species) and slightly drier forest regions.

In Europe they are restricted to the Atlantic Ocean fringes of the continent, notably in the Azores, Ireland, and western Great Britain, but one species (Hymenophyllum tunbrigense) locally east to Luxembourg, another (H. wilsonii) so far north as West Norway, Faeroes and South Iceland, while in North America, they are restricted (often occurring solely as gametophytes) to the humid eastern third of the continent and the rainforests of the Pacific Northwest.