Alstonia is the largest and most widespread genus of trees and shrubs in the subtribe Alstoniinae of the tribe Plumerieae of the Apocynaceae. Many of its species provide important timbers of commerce, and several species are used in traditional local medicine. The genus occurs in Central America, tropical Africa, and from the Himalayas and China to New South Wales in Australia, and has its centre of diversity in the Malesian region. This study aims to contribute to our still fragmentary knowledge of the tropical biodiversity, and provides a comprehensive survey of the macromorphological and pollen morphological diversity within the genus Alstonia (Chapter 2), a cladistic analysis based on this diversity (Chapter 3), a comparative wood anatomical study of three of the five sections with analyses of ecological and phylogenetic trends in the wood anatomical data (Chapter 4), and a full taxonomic revision of the entire genus (Chapter 5).