This monograph is an ecological study and complete revision of the genus Leochilus (Orchidaceae: Oncidiinae). Examination of living, liquid preserved, and pressed specimens was supplemented with fairly extensive study in the species‘ native habitats in Mexico, Central America, northern South America, and the Caribbean (more or less throughout the complete range of the genus). Various forms of laboratory investigations were conducted on many vegetative and floral features (most of which are illustrated). Chromosome counts are reported for L. carinatus (2n = 42), L. crocodiliceps (2n = 48), and L. oncidioides (n = 21). Twig epiphytism and the modiﬁcations of the Leochilus species for this habitat are discussed, including the psygmoid (fanshaped) juvenile habit, seed morphology, and epiphytic weediness. Pollination of Leochilus by lower hymenopteran vectors is described and discussed with special emphasis on one case of microsympatry. Five of the nine species in Leochilus are known to occur in mixed populations, yet no natural hybrids have been identiﬁed. Genetic identity appears to be maintained by adaptation for pollination by different sizes and types of insects. Contrary to historical placement, Leochilus is considered at most only a distant relative of Oncidium, with which it shares only the general vegetative, ﬂoral, and ecological features of the subtribe Oncidiinae. Leochilus is more closely related to a group of other twig epiphytes, including Capanemia, Comparenia, Ionopsis, Notylia, Papperitizia, and Rodriguezia, and is most closely related to Hybochilus leochilinus. A Groundplan-divergence Method cladogram is presented for Leochilus; four clades are identiﬁed, each with a number of parallel character states. The section on taxonomy includes a generic history and description as well as a key and descriptions, discussions, illustrations, and distribution maps for each species.