The history of Buddhism in Tibet begins with Bon. The Bon religion of Tibet was animistic and shamanistic, and elements of it live on today, to one degree or another, in Tibetan Buddhism.
Although Buddhist scriptures may have made their way into Tibet centuries earlier, the history of Buddhism in Tibet effectively begins in 641 CE. In that year, King Songtsen Gampo (d. ca. 650) unified Tibet through military conquest and took two Buddhist wives, Princess Bhrikuti of Nepal and Princess Wen Cheng of China. The princesses are credited with introducing their husband to Buddhism.
Guru Rinpoche and Nyingma
During the reign of King Trisong Detsen, which began about 755 CE, Buddhism became the official religion of the Tibetan people. The King also invited famous Buddhist teachers such as Shantarakshita and Padmasambhava to Tibet.
Padmasambhava, remembered by Tibetans as Guru Rinpoche ("Precious Master"), was an Indian master of tantra whose influence on the development of Tibetan Buddhism is incalculable. He is credited with building Samye, the first monastery in Tibet, in the late 8th century. Nyingma, one of the four major schools of Tibetan Buddhism, claims Guru Rinpoche as its patriarch.
According to legend, when Guru Rinpoche arrived in Tibet he pacified the Bon demons and made them protectors of the Dharma.
In 836 King Tri Ralpachen, a supporter of Buddhism, died. His half brother Langdarma became the new King of Tibet. Langdarma suppressed Buddhism and re-established Bon as the official religion of Tibet. In 842, Langdarma was assassinated by a Buddhist monk. Rule of Tibet was divided between Langdarma's two sons. However, in the centuries that followed Tibet disintegrated into many small kingdoms.
While Tibet was plunged into chaos, there were developments in India that would be keenly important to Tibetan Buddhism. The Indian sage Tilopa (989-1069) developed a system of meditation and practice called Mahamudra. Mahamudra is, very simply, a methodology for understanding the intimate relation between mind and reality.
Tilopa transmitted the teachings of Mahamudra to his disciple, another Indian sage named Naropa (1016-1100).
Marpa and Milarepa
Marpa Chokyi Lodro (1012-1097) was a Tibetan who traveled to India and studied with Naropa. After years of study, Marpa was declared a dharma heir of Naropa. He returned to Tibet, bringing with him Buddhist scriptures in Sanskrit that Marpa translated into Tibetan. Hence, is he called "Marpa the Translator."
Marpa's most famous student was Milarepa (1040-1123), who is remember especially for his beautiful songs and poems.
One of Milarepa's students, Gampopa (1079-1153), founded the Kagyu school, one of the four major schools of Tibetan Buddhism.
The Second Dissemination
The great Indian scholar Dipamkara Shrijnana Atisha (ca. 980-1052) came to Tibet by invitation of King Jangchubwo. At the request of the King, Atisha wrote a book for the the king's subjects called Byang-chub lam-gyi sgron-ma, or "Lamp to the Path of Enlightenment."
Although Tibet was still politically fragmented, Atisha's arrival in Tibet in 1042 marked the beginning of what is called the "Second Dissemination" of Buddhism in Tibet. Through Atisha's teachings and writings, Buddhism once again became the main religion of the people of Tibet.
Sakyas and Mongols
In 1073, Khon Konchok Gyelpo (1034-l 102) built Sakya Monastery in southern Tibet. His son and successor, Sakya Kunga Nyingpo, founded the Sakya sect, one of the four major schools of Tibetan Buddhism.
In 1207, Mongol armies invaded and occupied Tibet. In 1244, Sakya Pandita Kunga Gyeltsen (1182-1251), a Sakya master was invited to Mongolia by Godan Khan, grandson of Genghis Khan. Through Sakya Pandita's teachings Godon Khan became a Buddhist. In 1249, Sakya Pandita was appointed Viceroy of Tibet by the Mongols.
In 1253, Phagba (1235-1280) succeeded Sakya Pandita at the Mongol court. Phagba became a religious teacher to Godan Khan's famous successor, Kublai Khan. In 1260, Kublai Khan named Phagpa the Imperial Preceptor of Tibet. Tibet would be ruled by a succession of Sakya lamas until 1358, when central Tibet came under control of the Kagyu sect.
The Fourth School: Gelug
The last of the four great schools of Tibetan Buddhism,the Gelug school, was founded by Je Tsongkhapa (1357-1419), one of Tibet's greatest scholars. The first Gelug monastery, Ganden, was founded by Tsongkhapa in 1409.
The third head lama of the Gelug school, Sonam Gyatso (1543-1588) converted the Mongol leader Altan Khan to Buddhism. It is commonly believed that Altan Khan originated the title Dalai Lama, meaning "Ocean of Wisdom," in 1578 to give to Sonam Gyatso. Others point out that since gyatso is Tibetan for "ocean," the title "Dalai Lama" simply might have been a Mongol translation of Sonam Gyatso's name -- Lama Gyatso.
In any event, "Dalai Lama" became the title of the highest-ranking lama of the Gelug school. Since Sonam Gyatso was the third lama in that lineage, he became the 3rd Dalai Lama. The first two Dalai Lamas received the title posthumously.
It was the 5th Dalai Lama, Lobsang Gyatso (1617-1682), who first became ruler of all Tibet. The "Great Fifth" formed a military alliance with the Mongol leader Gushri Khan. When two other Mongol chiefs and the ruler of Kang, an ancient kingdom of central Asia, invaded Tibet, Gushri Khan routed them and declared himself king of Tibet. In 1642, Gushri Khan recognized the 5th Dalai Lama as the spiritual and temporal leader of Tibet.
The succeeding Dalai Lamas and their regents remained the chief administrators of Tibet until the invasion of Tibet by China in 1950 and the exile of the 14th Dalai Lama in 1959.