The present study deals with the taxonomy of a family of the brown algal order Sphacelariales in Europe. The taxonomy of this order is much influenced by the works of Sauvageau as published between 1900 and 1914. A short survey of the work on Spacelariales by him and his phycological predecessors is given in the introduction. The order Sphacelariales is described and its nomenclatural history is given. Other paragraphs deal with distribution, morphology and the used descriptive terminology, ecology, variability and culture studies, reproduction and life-history, systematic position and classification. In the notes on morphology the history of the descriptive terminology is incorporated, as well as discussions on the correct use of this terminology. Most technical terms are also included in the glossary, located near the end of this book. In the sections on ‘Form range and cultures’ and on ‘Reproduction and life-history’ the methods used for unialgal cultures and methods for chromosome counts are discussed. Also a review of life-histories in Sphacelariales is incorporated, as well as a discussion on the criteria used for the distinction of taxa and the delimination of the order. A key to the families concludes the treatment of the order. The family Sphacelariaceae, which is the largest and most cosmopolitan family of the order, is treated in a similar way. The two genera in this family, the monotypic genus Sphacella and the complex genus Sphacelaria, which contains four subgenera, seven sections and 16 species in Europe, are also treated in comparable paragraphs. Keys to the taxa and to ecological growth-forms (ecads) are given. In the paragraph on relationship of genera, subgenera, sections and species, several approaches for the construction of a classification are mentioned. The phyletic-cladistic approach, based upon methods developed by Hennig (1950), is discussed in detail. One conclusion is that the genus Choristocarpus cannot be considered to belong to a monophyletic group together with the Sphacelariaceae. Further it can be concluded that the Sphacelariaceae all belong to one group with a monophyletic origin. The monotypic genera Battersia, Disphacella and Chaetopteris have to be included into the genus Sphacelaria. Sphacella, however, is maintained as a monotypic genus. For nomenclatural reasons Sphacelaria reticulata (formerly Disphacella reticulata) must be chosen as type-species of the genus Sphacelaria. The descriptions of family, genera and sections are usually short, but the descriptions of the species are comprehensive and contain a formal description and a list of dimensions. The paragraphs on distribution start with summaries of coastal regions where the species occur. Each summary is followed by an extract of the list of collections and relevant references. Distribution maps are added. Full lists of collections and references for all species are published separately. Important taxonomic conclusions occur in Sphacelaria reticulata (was Disphacella reticulata (Lyngb.) Sauv.), in S. radicans (ecad libera found in the Baltic), in S. nana (= S. britannica Sauv.) which include S. saxatilis and which is different from S. rigidula (= S. furcigera Kütz.), in S. plumigera (unattached growthform = ecad pinnata, found in the Baltic), in S. mirabilis (was Battersia mirabilis Reinke ex Batt.), in S. fusca (different from S. rigidula), in S. cirrosa (includes S. bipinnata (Kütz.) Sauv. and S. hystrix Suhr ex Reinke which are incorporated amongst the five different ecads of the species) and in S. sympodiocarpa (which cannot be incorporated into one of the described subgenera). Most details of morphology are depicted.