The Karmapa heads the Karma Kagyu school of Tibetan Buddhism and holds the oldest continuously recognized reincarnation lineage in Tibetan Buddhist history, predating the Dalai Lama lineage by roughly three centuries. The first Karmapa, Dusum Khyenpa, lived from 1110 to 1193 and was a direct disciple of Gampopa, the founder of the Kagyu school's monastic tradition; before his death he is said to have left a letter predicting the circumstances of his own rebirth, establishing the pattern of prediction letters that later Karmapas and other Tibetan reincarnation lineages, including the Dalai Lamas, would also adopt.
The Karma Kagyu developed a network of monasteries across Tibet, most centrally Tsurphu Monastery, and the Karmapas were influential figures at the courts of Mongol and Chinese emperors during the Yuan and Ming dynasties, well before the Gelug school's Dalai Lama lineage rose to political prominence in the seventeenth century. The sixteenth Karmapa, Rangjung Rigpe Dorje, fled Tibet for Sikkim in 1959 alongside the wider Tibetan exodus that followed the failed uprising against Chinese rule, establishing Rumtek Monastery as the school's main seat in exile.
The Karma Kagyu is one of four principal schools of Tibetan Buddhism, alongside the Nyingma, Sakya and Gelug, and is further organized into the Four Great and Eight Lesser sub-lineages historically associated with different monastic seats across Tibet and the Himalayan borderlands. The Karmapa's traditional role as a unifying figure across these sub-lineages gives the office a significance within the Kagyu tradition broadly comparable to the Dalai Lama's within the Gelug school, even though the two lineages developed independently and belong to different schools.