Plants terrestrial to rarely epiphytic. Rhizomes short to long-creeping, decumbent to erect, bearing scales at apexes. Stipes in cross sections at bases reveal ends of 2 ribbon-shaped vascular bundles (appearing as linear and parallel lines) that unite, forming a single U-shaped bundle in upper stipes, grooved, sulci not running continuously into axis of next order. Blades 1-pinnate to 3-pinnate-pinnatifid, usually 1-pinnate-pinnatifid. Rachises grooved adaxially or not, grooves (if present) not continuous with grooves of costae, with short, unicellular, sharp-tipped hairs. Pinna hairs on adaxial surfaces similar to those on rachises, sinuses between two pinna lobes often with a translucent, cartilaginous membrane at bases, aerophores often present at abaxial bases of pinnae. Veins free and forked, pinnately branched in lobes, sometimes joining below sinuses to form excurrent vein to sinuses. Sari round. Indusia present or not.

One of the largest fern families, with about 1,000 species worldwide. Represented in Hawai 'i by seven endemic, two indigenous, and two naturalized species as well as at least two hybrids. There are natural groups of species in the Thelypteridaceae, but whether to recognize them as genera or subgenera is a subject of continuing debate. Some authors place all of the species in only a few genera, but others recognize as many as thirty-two genera. Some include all the species in the genera Cyclosorus, Thel)pteris, and Pseudophegopteris; some use a sub-generic classification such as Thelypteris (Christella) dentata; and some recognize the sub-genera as genera. The classification used here follows that used by Holttum (1977) in his extensive monograph of the Pacific and Australasian ferns. Holttum's genera work very well with Hawaiian species and have been used extensively in the organization of herbarium collections, in unpublished checklists, and in publications regarding this fern group. When Holttum's classification is applied worldwide the margins of the genera may not be as clear. Future studies and a detailed revision of the Thelypteridaceae worldwide will help address this problem.

Ferns of the terrestrial family Thelypteridaceae may be recognized by stipes revealing two crescent-shaped, linear vascular bundles on cross section at the base; the presence of short, 1-celled, pointed hairs on the surfaces of the rachises, costae, and blades; and usually the presence of a translucent, cartilaginous membrane at the base of each sinus.