Starting from the recently produced Italian Habitats interpretation Manual (http://vnr.unipg.it/habitat), a thorough diagnosis of the Annex I Habitats recorded in the Italian territory is reported, selecting the alliance level as the most suitable standard phytosociological rank for a detailed nationwide overview. In order to solve a number of well-known problems, arising from the definitions reported in the European EUR/27 Manual, each Habitat has been assigned a short, exhaustive although concise diagnostic sentences which contains its most significant ecologic features, with specific reference to the peculiarities of the Italian territory. Biogeographic, synecological (mainly bioclimatic, morphological, geologic/edaphic), structural and floristic characteristics of each Habitat are pointed out. When the rarity or vulnerability status has been considered worthy of priority at the national scale, it has been indicated. The 131 Habitats listed for Italy, which represent only partially the Italian biodiversity, have been referred to 268 alliances. A complete syntaxonomic framework is reported in order to offer a robust although still improvable phytosociological ground. The paper aims at standing as a reference document for Natura 2000 knowledge and management in Italy; it also represents a large-scale, expert-revised tool to allow supra-national comparisons and support future implementation of the Annex I.