| Summary: | In the Northern Hemisphere, <i>Plagiothecium cavifolium</i> is currently one of the most widely distributed species. This taxon has been described as extremely variable for decades, but the reasons for this variability have not been investigated in detail. The analysis of original materials and diagnoses, as well as a detailed analysis of the history of names considered as synonyms of <i>P. cavifolium</i> <i>sensu</i> <i>lato</i>, showed that in terms of qualitative and quantitative characteristics, a number of the names of this complex differ significantly from the diagnosis of <i>Hypnum cavifolium</i> (basionym of <i>P. cavifolium</i>). The most important features distinguishing individual taxa include: julaceous stems; imbricate leaves, their symmetry, concavity; serration of leaf apices; the length of the cells from the middle part of the leaf; and the orientation of the capsules. Thus, the research conducted within <i>P. cavifolium</i> <i>sensu</i> <i>lato</i> made it possible to distinguish seven separate taxa: <i>P. cavifolium</i> (= <i>P. cavifolium</i> <i>sensu stricto</i>), <i>P. flaccidum</i>, <i>P. tenue</i> (being a new combination), <i>P. ikegamii</i>, <i>P. subjulaceum</i>, <i>P. sakuraii</i> and <i>P. otii</i> (four resurrected species). In addition, the analysis of original materials and the diagnosis of several taxa allowed them to be excluded from the described complex, and here we propose their synonymization with other taxa, such as <i>P. longisetum</i> and <i>Hygrohypnum luridum</i>. Photographic documentation and a key to distinguishing species within the described complex are attached. For two names (<i>P. sakuraii</i> and <i>P. succulentum</i> var. <i>longifolium</i>) lectotypes are proposed.
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