A complete revision is given of the Indo-Malesian genus Cratoxylum. The subdivision of the genus into 3 sections, as given by Engler (1925) and Corner (1939), has been found correct. The characters by which these sections are discriminated concern the interpetiolar scars on the twig, the type of venation, the occurrence of petal-scales and their shape and size, the shape of the hypogynous scales alternating with the three staminal phalanges, and whether the seeds are surrounded by a wing or are winged on one side only. Each section contains two species. A concise review has been given about the history of the genus, an account of the uses made of it, and the ecology. It appears that almost all species can act as pioneers for which they are fully qualified by their euredaphic requirements and early and abundant seed production. Though some species, notably C. arborescens and C. glaucum, are evergreen, and C. formosum is deciduous, the others are not specifically one or the other way round; in C. maingayi Corner noted that sometimes particular branches shed their leaf. There is no major correlation between leaf-shedding species and seasonal climate regime. A brief summary is given on the morphology, in particular the structure of the inflorescence. Two points have not become entirely clarified, for example, whether all species are always heterodistylous: this should be studied further in the field. Another point is the want of more exact data on the degree in which the ovary is incompletely celled which seems to differ from one species to another, and to compare this with the ovary of the allied Madagascan genus Eliaea (vide infra). C. glaucum excepted all species show a distinct variability. In three species this is grading and no infraspecific taxa can be distinguished in a key on the basis of herbarium material. Within Malesia the variability is more or less of a geographical character and could be classified under clinal variation. This is also the case in C. sumatranum and C. formosum, within which I can distinguish, however, three and two infraspecific taxa respectively, which accordingly have been given racial rank, that is, as subspecies. Within each a still finer distinction might be feasible of subsubraces which mostly coincide with separate islands of Malesia. The closest related genus is the monospecific genus Eliaea Camb. from Madagascar with which it forms a tribe of the Guttiferae. With regard to the exact structure of the ovary claimed to be distinct from that of Cratoxylum a close comparative-morphological study should be made of the ontogeny of the ovary in Eliaea and all species of Cratoxylum. Except from taxa described by Loureiro, Jack, and Blanco, of whose names no types seem to exist, I have for the first time examined type material of all names and checked their detail floral characters which were often not mentioned in the original diagnoses. A listed account of all numbered material on which this study was based will be published in a separate ‘Identification List’.