Campylopus is a large, taxonomically complex group of worldwide distribution. Due to great variation in nearly all its characters the generic limits of the genus are diffuse. In general, Campylopus consists of medium to large, dioicous mosses with stiff, erect to spreading (generally unbordered), linear leaves having broad, excurrent costa approximately 1/2 to 2/3 the leaf base width. Many species have costae with a ventral layer of variously enlarged hyaline cells (hyalocysts). Sporophytically, the cygneous setae, capsules that lack stomata, and dicranaeous peristome teeth are distinctive. The calyptrae are cucullate and frequently ciliate at base.
In the typical sense Campylopus is easy to recognize, but the genus harbors species whose characters grade into those of Atractylocarpus, Dicranodontium, Dicranella, Campylopodium, Paraleucobryum, and Pilopogon (see Allen 1989). In the following treatment Campylopus is divided into two groups on the basis of the form of the costal cells ventral to the guide cells.