The family Orthotrichaceae (Musci) in North America, north of Mexico is divided into sixty-five taxa in eight genera. Fifty-five species, eight varieties and two forms are recognized. Keys are presented to the subfamilies, genera, subgenera, and species. Each taxon is treated by a complete list of synonymy, a taxonomic diagnosis, and a description of the habitat and distribution. The species are fully illustrated and their distributions plotted on maps. Orthotrichum flowersii, Orthotrichum pallens var. crenulatum, and Orthotrichum pallens var. carothersii are described as new to science, and several new combinations are made, as follows: Orthotrichum strangulatum fo. lescurii,, Orthotrichum laevigatum fo. macounii, Zygodon viridissimus var. apiculatus, and Macrocoma sullivantii. The family is considered as a member of the double peristome series (Diplolepideae) and is most closely related to the families Ptychomitriaceae and Rhachitheciaceae. Amphidium, which has no peristome, is excluded from the family and because of other resemblances is placed in the Rhabdoweisiaceae. The family Grimmiaceae, although superficially similar, is considered a member of the single peristome series (Haplolepideae). The suprageneric categories of the family are revised, with four subfamilies being recognized, Drummondoideae, Zygodontoideae, Macromitrioideae, and Orthotrichoideae. The morphology and cytology of each major group are discussed. The genus Orthotrichum is treated in detail, including discussions on its ecology, distribution, and morphology. The species of Orthotrichum are characteristically found on tree trunks and on rock faces in xeric habitats in the temperate portions of the world. The genus has evolved from moist habitats into xeric habitats and the adaption of the species into these dry habitats has involved the change of at least ten characters. Based on phyletic data, a new classification of the infrageneric taxa is presented. The groundplan/divergence method developed by W. H. Wagner is used to express graphically the probable phylogeny and relationships of the species. The ecological and distributional data correlate well with the morphologically based ground plan. The taxa with "arcto-tertiary" distributions average more primitive than the taxa with endemic North American distributions, with the latter species terminal in evolutionary specialization. In addition, a geographic origin in the Southern Hemisphere and subsequent migration of the genus into the Northern Hemisphere is hypothesized.