The present study deals with those pyrenomycetes which occur on the gametophytic stage of bryophytes. The special part is devoted to the systematic arrangement and descriptions of the species and genera included; the general part discusses those problems which are specially connected with the bryophilous habit. Although pyrenomycetes are the main concern, reference to other fungal parasites of mosses is frequently included in the text. 1. Of the 123 known taxa, roughly assigned to 33 genera, 89, in 21 genera, are here described in detail and a key to their identification presented. Most of the taxa belong to the Dothideales, a smaller number to the Hypocreaceae and Lasiosphaeriaceae. 2. Nine genera, 62 species and three varieties are newly described. New combinations for a further 24 taxa are proposed, several are placed as synonyms. Numerous new species are to be expected in the course of future research and even in Europe many taxa are not yet completely understood. 3. The genera Lasiosphaeria, Leptomeliola, Pleosphaeria, Pleospora and Teichospora have been omitted. All known records and literature for all the other species have been surveyed and listed. Personal collections, many of which represent first records, have increased the number of samples studied by 85 percent to 1 300 in all. 4. Very great difficulties have been encountered in assigning species to the correct genera. It is supposed that some of the bryophilous fungi have evolved independently on their unique substrate and this substrate cannot be in any way compared with that found in other higher plants. The suggested systematic arrangement cannot at this stage be considered as more than a tentative interpretation. 5. 250 species of bryophytes in 110 genera are indicated as hosts. Probably all liverworts and mosses may be considered as potential hosts. No lager moss associations appear to be without some fungal associates. 6. Long-lived bryophytes without a quick alternation of the gametophytic stage have been shown to provide the most favourable substrates. Polytrichum is the most favoured host genus with 15 recorded fungal associates. On Plagiochila porelloides 7 fungal species have been recorded. In one Prionodon bolivianus tuft no fewer than 19 ascomycetes and fungi imperfecti were recorded. 7. Those fungi growing on bryophytes have been found to be very frequent and in certain regions bryophytes appear to be universally infected. In view of their abundance their neglect is inexplicable. 8. With few exceptions bryophilous fungi are unable to colonize other substrates. As well as nonspecific taxa which occur on a wide range of hosts, there are specialised taxa confined to a single host species. It is suggested that inoculation experiments could clarify some of the questions regarding the choice of host. 9. Observations on the biological nature of the associations indicate that both saprophytic, semi- and truly parasitic species are discernable. In addition, several lichenized species have been noted, which are associated with both bryophytes and algae at the same time. 10. The reduction in the size of the fruit body, vegetative growth and the ability to produce fruits at any time of the year, as well as a marked resistance to dry periods, are to be regarded as specific adaptations for the mode of living of these kinds of fungi.