The biodiversity of the Himalaya province is extraordinarily high. The eastern part of Himalaya embraces the zones of highest biological diversity, with more than 4,000 species of vascular plants per 10,000 km2 area. This is exceptionally high even in the global context. In the larger part of Himalayan province, the vascular plant species is above 3,000 per 10,000 km2 area. (Fig. 1.4) Nearly 30% of the forest plants are uniquely Himalayan, and are not found anywhere else in the world. These include certain species of Oak, Rhododendron and Pine. About 125 plant species are wild relatives of cultivated plants, including cereals, legumes and nuts. These constitute valuable gene-pools which can be used in future for the improvement of crops. Of the 1,500 species of angiosperms known in India, 20 to 49% occur in the Himalayan province. More than 30% of these angiosperm plants are entirely endemic and economically quite useful. Nearly 7,020 species or 54% of the Indian Fungi have been reported from this mountain domain. About 1,159 out of the total 1,948 Indian lichens, occurs in the Himalayan world. Likewise 23% or 115 species of Indian bryophytes grow on the Himalayan slopes. The composition of plant assemblages of the wetter eastern Himalaya is very different from that of the relatively drier western Himalaya. (Fig. 1.5) Within the western sector, the forest displays a tremendous diversity of floral characters, from the dense evergreen tropical vegetation of the torrid Bhabhar and Siwalik belts, through mixed deciduous trees with savannah grasslands in the salubrious middle mountain, to the sparce arctic type plants in the cold high mountain belt. In the western Himalayan domain the following floral assemblage is seen. The forests of the Bhabhar Siwalik terrain are dominated by Saal (Shorea roburta), Khair (Acacia catechu), Sheesham (Dalbergia sissoo), Haldu (Adina cordifolia), and Sain (Terminalia tomentosa) along with infinite varieties of shrubs and grasses. In the lower altitudes of Lesser Himalaya, the Chirpine (Pinusro xburgii) grows luxuriously on dry slopes with poor soil condition, while forests of Oaks (Quercus leucotrichophora, Quercus incana), Rhododendron (Rhodondron arboreum) and Alder (Alnus nepalensis), cover the moist slopes with good soils. The productivity of this vegetation is quite high. The dry biomass being as much as 20 tonnes per hectare per year. The higher altitudes of the lesser Himalayan mountains have forests characterized by Quercus semecarpifolia, Quercus dilata, Pinus wallichii Acer etc (Rau, 1975). The great Himalayan domain is covered with forests dominated by Silver Fir (Abies pindrow) Bhojpatra or Birch (Betula utilis) Fir (Abies spectabilis), stunted Rhododendron (R. campanulatum), and Junipers (Juniperus squamata, J. indica). Still higher, the forests give way to alpine meadows which have an infinite variety of flowering grasses. (Chopra, Nayar & Varma,1969) In the eastern sector the assemblage of important plants is characterized by the low altitude (<800m), tropical deciduous forests dominated by Shorea robusta which give way to subtropical forests with Schima - Castanopsis in the range of 800 to 1,800m. Ascending the gradient from 1,800m to 3,000m, the composition of the forests changes from Quercus lamellosa, Quercus pachyphylla to Tsuqa dumosa - Quercus semecarpifolia assemblage. In the altidunal zone of 3,000 to 4,000m, the forests are dominated by the Abies densa - Betula utilis assemblage. Rhododendron anthopogon occurs at 4,000 to 5,500m altitudes. And, above 5,500m, grow the species of Delphinium. Eastern Himalaya, is rich in bio diversity. Nepal, Bhutan, Sikkim and Assam regions are the biggest reservoirs of traditional medicine. (Zurick & Karan,1998). Bhutan with rich biodiversity is one of those regions in Himalaya where the tribal populations and forest dwellers form a considerable part of the population. The land locked mountainous kingdom of Bhutan constitutes an important segment of the eastern Himalaya. It is flanked in south-west, south and east respectively by the states of Sikkim, West Bengal, Assam and Arunachal Pradesh of India and in north by Tibet of China (Fig. 1.6). Bhutan is believed to have derived its name from Sanskrit word "Bhu-Uttan" or "Bhot-ant". The former means sudden rise in elevation, while the latter conveys end of Tibet. For locals the name of this fabulous country is "Druk Yul - Land of the Thunder Dragon". Bhutan has an area of 46,500 It lies between latitudes 26º40' and 28º40' north between longitudes 88º-45' and 92º-10' east. (Bhargava, 1995). Several plants in this area have potential of better economic exploitation e.g. Nardostachys grandiflora, Podophyllum emodii, Polygonatum verticillatum, Ephedra gerardiana, Artimisia vulgaris and many others. The present investigations, have been undertaken with the objective of characterization of biodiversity of the hilly areas. The different zones of sub-Himalayan region and Himalayan region, with special reference to Bhutan, were broadly characterized for vegetation types. Some of the plants have been dealt with in detail for their characterization and pharmacognostical parameters. Ethnobotanical attributes of the plants commonly utilized by the native people have also been studied. The exploration in remote corners of eastern Himalaya which is known for its rich biodiversity and tribal people, is considered to be the need of the hour. The characterization of the biodiversity of the region with special reference to medicinal plants and the plants with other ethnobotinical applications, is considered to be of paramount importance.