Euphorbiaceae is one of the fifteen most important families in the middle Caquetá region, containing economically important species such as rubber trees (Hevea spp.), cassava (Manihot esculenta) and the oil producing inchi (Caryodendron orinocense). There are many other Euphorbiaceae species utilized by indigenous communities, whose knowledge of their use is disappearing rapidly. Despite the importance of this family in terms of diversity and potential use, very little research has been done on Euphorbiaceae species in Colombia. This book contains a taxonomic description of 36 genera and 64 species, including nine new ones. In most cases the descriptions are accompanied with illustrationc. Where available, information is included on their distribution, use and ecology. A synoptical key was constructed to identify the Aracuarian species of Euphorbiaceae. This work is the result of a study carried out in the period 1990-1993 by the National Science Institute on the National University of Colombia.