As a consequence of its conspicuous sterile flowers, the genus Hydrangea is since many centuries a popular ornamental plant in China and Japan. Following the introduction by eighteenth and nineteenth century plant hunters such as Thunberg and von Siebold, several species became cultivated in Europe and America, resulting in more than 1000 cultivars. However, the relationships between the botanical species are poorly known. The number of accepted species depends from the authors and varies between 23 and 73. Although Hydrangea species are easily recognizable, morphological and molecular studies prove that the genus is at least paraphyletic and that about eight satellite genera are very closely related. Cladistic analysis of the sequenced genes (matK and rbcL) results in polytomous phylogenetic trees and the statistical support of the currently available results is too low to conclude about the phylogenetic relationships between the species. Although it turns out to be obvious that many questions concerning the relationships in the genus Hydrangea and the tribus Hydrangeeae remain unanswered, further research has surprisingly not been performed following the first orientating phylogenetic and molecular studies.The most recent complete world revision of the genus dates back to 1957 (McClintock) and is mainly based on the study of herbarium material. However,  the identification key of this work only is practicable with some difficulty as not all variation has been included and the species delimitation is vague. The combination of detailed morphological and molecular studies offers new possibilities to resolve the obscure relationships in this genus.Likewise, the disjunct distribution of the genus Hydrangea in the temperate biomes of eastern Asia, eastern North America and the tropics of both hemispheres remains unelucidated.The national Hydrangea-collections in Belgium and Germany and the collections of nurserymen and collectors in France and the United Kingdom make a good starting point for this research project. This study will answer current issues of interest concerning the phylogeny of the genus Hydrangea s.l., just as resolving the affinities in some species complexes.Within this project, we will resolve the relationships between the botanical species and the infraspecific taxa of the genus Hydrangea with the revision of McClintock (1957) as point of departure. As a consequence of the polyphyletic nature of the genus Hydrangea, this will result in a better understanding of the phylogeny of the tribus Hydrangeeae in de familie Hydrangeaceae (nl. Broussaisia, Cardiandra, Decumaria, Deinanthe, Dichroa, Hydrangea, Pileostegia, Platycrater en Schizophragma).