The state of Arunachal Pradesh is the largest state in the northeast. The state stretches from the snow-capped eastern Himalayas, at an altitude of 6,000 meters, to the plains of the Brahmaputra valley. Arunachal Pradesh is surrounded by Bhutan, China and Burma. Assam lies to its south. Kangto amd Nyegi Kangsang, and the Gorichen peak are some of the highest peaks in this part of the Himalayas.

The major rivers that flow through the state are Kameng, Subansiri, Siang (later Brahmaputra in Assam), Lohit and Tirap. The rivers have created broad valleys in these areas. The Brahmaputra enters Arunachal Pradesh from Tibet and flows into Assam from where it goes down to Bangladesh before falling into the Bay of Bengal. The capital of the state is Itanagar, which is situated in the foothills of the Himalayas.

The climate varies with elevation. Areas with very high elevation enjoy an alpine or Tundra climate. Areas near to the Middle Himalayas experience a temperate climate. Areas at the sub-Himalayan and sea-level elevation experience a humid sub-tropical climate, with very hot summers and mild winters. The state recieves very heavy rainfall of 2,000 to 4,000 mm. Due to the heavy rains, the forests are luxuriant in growth and the rain also accounts for the large number of rivers and lakes in the region. The rainy seasons is from May to September.