This paper contains the original diagnoses of new syntaxa, typifications of existing names of syntaxa, and other decisions of nomenclatural relevance which have proved to be necessary within the project 'Plant communities of Mecklenburg-Vorpommern and their vulnerability'. The underlying syntaxonomic concept is documented in DENGLER & BERG (2002). In BERG et al. (2001 b, 2003) we have discussed in detail the syntaxonomic system as it applies to the federal state of Mecklenburg-Vorpommern in NE Germany. In the introductory sections we deal with some methodological questions and explain the mode of presentation adopted in the special section. We follow strictly the rules of the International Code of Phytosociological Nomenclature (ICPN), but have recognised six aspects in which the present version of the ICPN is unclear or contradictory. For these cases we suggest reasonable solutions that are followed in the special section. The special section of our work will be published in two parts. The present paper deals with nine phytosociological classes belonging to the herbaceous terrestrial vegetation (Polygono-Poetea annuae, Sisymbrietea, Stellarietea mediae, Calluno-Ulicetea, Koelerio-Corynephoretea, Festuco-Brometea, Molinio-Arrhenatheretea, Trifolio-Geranietea, Artemisietea vulgaris). Altogether 17 new syntaxa are validly published (including validations of previously used names and changes in rank). One name of a syntaxon is corrected due to a taxonomic error and 151 further syntaxa are typified. Nomenclatural questions are discussed if appropriate and the reasons given when applications concerning nomina ambigua, conservanda, inversa or mutata are proposed. The most important syntaxonomic novelties are the following: Subdivision of the Sisymbrietea into several orders, of which the Sisymbrietalia and the Conyzo canadensis-Brometalia tectorum ord. nov. occur in Central Europe - Subdivision of the Stellarietea mediae into the Aperetalia spicae-venti, the Dicranello staphylinae- Stellarietalia mediae ord. nov. and the Papaveretalia rhoeadis ord. nov. -Subdivision of the Koelerio-Corynephoretea into the two subclasses Koelerio-Corynephorenea subcl. nov. and Sedo-Scleranthenea subcl. nov. - Establishment of an alliance Filipendulo vulgaris-Helictotrichion pratensis all. nov. which contains floristically impoverished semi-dry grasslands in the southern Baltic area - Subdivision of the Molinio-Arrhenatheretea into the two subclasses Arrhenatherenea subcl. nov. and Molinio-Juncenea subcl. nov. - Subdivision of the Trifolio-Geranietea into the two subclasses Melampyro-Holcenea subcl. nov. und Trifolio-Geranienea subcl. nov. (containing the Origanetalia vulgaris s. str. and the AnthericoGeranietalia sanguinei ord. nov.) - Subdivision of the Artemisietea vulgaris into the four subclasses Epilobienea angustifolii, Lamio albi-Urticenea dioicae subcl. nov., Agropyrenea intermedio-repentis subcl. nov. (containing the Rubo caesii-Calamagrostietalia epigeji ord. nov. and the Agropyretalia intermedio-repentis) and Artemisienea vulgaris.