Seventeen new species of Gyalectidium have been discovered in various parts of the world, and those unexpected findings formed the starting point for a survey of the taxonomy and ecogeography of the genus. The following species are described as new in this paper: G. areolatum Ferraro & Lucking (Neotropics), G. atrosquamulatum Lucking & Kalb (Kenya), G. australe Lücking (Australia), G. conchiferum Lücking& Wirth (Chile), G. den-ticulatumLücking (Costa Rica), G. fantasticumFerraro & Lücking (Neotropics), G. flabellatumSérus. (Australasia), G. fuscumLücking & Sérus. (Africa and Papua New Guinea), G. gahavisukanumSérus. (Papua New Guinea), G. kenyanum Lücking & Kalb (Kenya). G. laciniatum Lücking (Costa Rica), G. maracae Lücking (Neotropics), G. membranaceum Sérus. & Lücking (Canary Islands), G. minus Sérus (Canary Islands and southern Italy), G. novoguineense Sérus. (Australasia), G. puntilloi Sérus. (south-west Europe), and G. verruculosum Sérus. (Australasia). Calenia microcarpa Vzda [Syn.: Bullatina microcarpa (Vda) Brusse] is included in Gyalectidium as G. microcarpum (Vzda) Lücking, Sérus. & Vzda comb, nov., and G. catenulatum (Cavalc. & A. A. Silva) Ferraro, Lücking & Sérus. is treated as a species different from G. ftlicinum. Gyalectidium corticola Henssen is transferred to Calenia as Calenia corticola (Henssen) Ferraro, Lücking & Sérus. comb. nov. A key to all 29 accepted species of Gyalectidium is provided. The infrageneric phylogeny is constructed by means of a phenotype-based cladistic analysis, and the systematic affinities of the genus are discussed, accompanied by notes on the distribution and ecology of the species. Apothecia are not yet known in several species, including new ones.