This monograph details what is known about Timonius in Borneo, a group in the extremely hyper-diverse coffee family Rubiaceae, that no one has studied since some scattered initial documentation in the past few centuries. Now, all Bornean territories are much more explored, botanically collected from, and ‘developed’. Still, very few studies have been made in Southeast Asia for complex and highly diverse plant groups which can illuminate our understanding of plant diversity and classification, and contribute to a more accurate assessment of plant lineage development and evolution. This is a first step, of many such first steps that will be needed, in addressing the incredible plant richness of the Malay Archipelago, a recognised region of spectacular biological richness. This account documents 94 species of Timonius, a genus of mostly treelets to small trees still poorly understood throughout Southeast Asia, of which 68 species are newly recognised. Some 96% of the genus is endemic to Borneo, not known beyond this large, physiographically and geologically diverse tropical island, a centrepiece in the biogeography of a vast, biologically diverse region of our globe. The book includes discussions on the morphology, biology and reproductive ecology of Timonius and related plants, and is illustrated by 294 figures and 58 botanical line-drawings.