A systematic investigation of the pleurocarpous moss genus Plagiothecium Bruch & Schimp. was performed as a part of the author’s PhD study at the University of Copenhagen. Plagiothecium is mostly a well-defined genus, but the species limits and the interspecific relationships are poorly understood. The relationships between Plagiothecium and other similar pleurocarps are also speculative. In this study, DNA sequence data were used for reconstructing the phylogeny of the genus. Based on the results of a preliminary survey of 16 DNA regions, three regions were chosen for genetic markers: plastid trnK-psbA and rpl16 intron, and nuclear ribosomal ITS. Parsimony analyses of DNA sequence data from all three regions were performed to generate phylogenies for 110 specimens of Plagiothecium and similar mosses. Some sequence data were obtained for an additional 76 collections, yielding an rpl16 phylogeny for 147 collections, and an ITS phylogeny for 233 collections that included 88 publically available sequences, mostly from extrageneric hypnalean taxa. In conjunction with the DNA study, a microscope study of over 2000 herbarium specimens was performed. The results of the DNA and herbarium studies were combined to refine the taxonomy of the genus as a whole. A number of new taxa are described, several nomenclatural novelties are proposed, and an improved sectional classification of the genus is offered. A synopsis of the genus is also provided as a contribution toward a worldwide revision.