Abstract. Delforge, P. 2013 - Contribution to the knowledge of the Orchids of the island of Skyros (Northern Sporades, Greece) [Natural. belges 94 (Orchid. 26): 165-244]. A presentation of the geology, the geomorphology, the climate, the vegetation, the human impact over the landscape, and the important isolation of the island of Skyros, factor of endemicity, is provided. A review of the past and present studies and research about its orchids is presented. It appears that 26 orchids species were known for Skyros before 2013 (Table 1). Research on Skyros in March 2011 has revealed 29 orchid species for the island, bringing their number up to 37 (Table 1). For the first time, the whole flowering season of orchids is viewed for Skyros, with the integration of the results of three previous research, published (Biel et al. 1998) or not, respectively made in April 1997, May 1997, and April 2003. First mentions are made for Ophrys achillis (sp. nova), O. ceto, O. creutzburgii, O. leochroma, O. lycomedis (sp. nova), O. cf. mesaritica, O. parosica var. parosica, O. pelinaea, O. phryganae, O. polycratis, O. scyria (sp. nova), O. sepioides, O. sitiaca s. str., O. villosa s. str., and Vermeulenia papilionacea var. aegaea. Except O. bombyliflora, O. sicula, and O. villosa, all the 29 species seen in March 2011 were scarce, and frequently represented by few individuals. After careful examination of ÒcriticalÓ plants as well as photographies, published or not, it appears that old or recent mentions of orchids, made for Skyros, are questionable or erroneous. Mentions of O. fusca and late-flowering O. sitiaca concern probably O. pelinaea, a mention of an occasional hybrid between O. israelitica and O. omegaifera concerns O. scyria (sp. nova), mentions of O. oestrifera and O. scolopax concern probably, respectively O. sepioides and O. ceto, mentions of O. tenthredinifera concern O. villosa, O. lycomedis (sp. nova), and O. leochroma. A list of 37 species and 4 hybrids, and 40 distribution maps, based on previous localizable records, published or not, and on the presence in March 2011 in 52 UTMwgs84 squares of 1 km × 1 km, are provided as well as a list of 68 sites prospected from March 1 to March 25 2011.Key-Words: Flora of Greece, Aegean, Northern Sporades, Skyros; Orchidaceae, Ophrys achillis (sp. nova), Ophrys lycomedis (sp. nova), Ophrys scyria (sp. nova), O. ceto, O. creutzburgii, O. leochroma, O. cf. mesaritica, O. parosica var. parosica, O. pelinaea, O. phryganae, O. polycratis, O. sepioides, O. sitiaca s. str., and O. villosa s. str, Vermeulenia papilionacea var. aegaea.