The genus Heracleum (Apiaceae) includes 65-70 species, and is distributed across the Northern Hemisphere from North America to East Asia (Pimenov & Leonov, 1993). The Sino-Himalaya region (Nepal eastwards to SW China) is one of the most important distribution areas of Heracleum (with 10 species in the Himalaya and about 30 species in China). This thesis integrates a series of studies on the systematics of Sino-Himalayan Heracleum including allied taxa and a taxonomic revision of Nepalese Heracleum species. The taxonomic value of the various morphological characters used in Heracleum is generally discussed and potentially important fruit morphological characters investigated alongside. Several characters were found to be distinctive, constant out of taxonomic importance. These included extent of development of the lateral wings, presence/absence of epidermal hairs, hair types, and the transverse sectional arrangement of the mericarp, including position of sclerenchyma of vascular bundles, rib development, pattern of endosperm, distribution of vittae and thickening of wing margins. Based on these characters, Heracleum was divided into at least 5 types that are also supported by molecular data (ITS DNA sequences). In order to evaluate the monophyly of Sino-Himalayan Heracleum, and clarify the species relationships within this group, sequences of ITS, rps16 intron, and psbAtrnH spacer of Heracleum and related genera were studied. The resulting data matrix was analyzed using Maximum Parsimony and Bayesian analysis. Both ITS and rps16 intron regions show a good resolution of phylogenetic relationships down to species level. However, although psbA-trnH region has been successively tested in other plant families, it failed to resolve relationships among the major Heracleum clade. Only higher level resolution was successful. The results of our comparative phylogenetic study confirmed that the “Major Heracleum clade” (Sensu Downie) remains monophyletic. Within this clade at least six major lineages are defined: Heracleum, Malabaila-Pastinaca, Zosima, Semenovia, Tordylium, and Tordyliopsis. The Sino-Himalayan Heracleum lineage is seen to comprise three distinct sub-lineages (Heracleum group I, II and III) including several European Heracleum species. Heracleum is thus not monophyletic. Generic segregates of Heracleum proposed in past classifications,such as Tetrataenium, can be correlated with these subclades. These phylogenies also helped resolve the taxonomic placement of some enigmatic taxa such as Tordyliopsis brunonis and Heracleum millefolium. These molecular data provide valuable characters for inferring relationships within the Sino-Himalayan Heracleum. A taxonomic account of the species of Heracleum in Nepal is presented including a key to identification of the species is produced. A reinterpretation of the taxonomy of Sino-Himalaya Heracleum is presented based on these results including an account of the Nepalese species.