This title is monograph 103 of the Flora Neotropica series; it is the recipient of the 2008 Augustin-Pyramus de Candolle Prize for Best Monograph, awarded by Societe de Physique et d'Histoire Naturelle de Geneve (SPHN). Foliicolous Lichenized Fungi treats an enigmatic group of organisms: lichens that grow on living leaves of vascular plants in tropical rainforests of the New World. It covers more than 600 species (about 70% of all species occurring worldwide) in more than 70 genera, 23 families, and 8 orders. Leaf-dwelling lichens are unusual in that they are small and have very short life-cycles, and representatives can be found in almost all major lineages of lichenized fungi. Up to 50 species can be found on a single leaf the size of a hand, and up to 300 species within a hectare of tropical rainforest. No other lichen group exhibits such extraordinary diversity. All taxa are keyed out and each species is treated in detail, including photographic illustrations and line drawings. The book features a lengthy introductory chapter covering the history of foliicolous lichen research, morphology and anatomy, taxonomy and systematics, evolution and phylogeny, ecology and biogeography, interactions with other organisms, and potential applications of these tiny organisms.