The sole object of the present work is to furnish a monograph dealing exclusively with the species of the genus Psathyrella (exclusive of Lacrymaria, see Chapter VI) reported from the Netherlands, France and the British Isles. Ever since 1958 we intensively collected, described, depicted and stored in our herbarium these species from many parts of the Netherlands and later studied the exsiccata microscopically. From 1960 on we did the same practically every year for some three weeks in many parts of the British Isles, often during the annual forays of the British Mycological Society, and particularly in Wales and Scotland. Moreover through the valuable aid from Dr. D. A. Reid, Dr. D. N. Pegler and Dr. R. Watling and the information supplied by the ‘New Check List of British Agarics and Boleti’ (Dennis, Orton & Hora, 1960) we became very well acquainted with the British species of Psathyrella. Mr. H. Romagnesi’s vast knowledge of and experience with the French species of Psathyrella and the great co-operation between him and us resulted in our becoming extensively informed about the French species of the genus. Our frequent exchanges of information and exsiccata even led to Romagnesi’s discovery of a new species (P. phegophila Romagn.) in his own herbarium, which he very kindly publishes in the present work. The results of our observations on Psathyrella in the three countries of course were compared with those published by A. H. Smith in his monograph on the North American species, hitherto the only monograph of this genus.