The Komnenoi originated in Paphlagonia, perhaps from the Kastamonu castle, which is possibly a corruption of Castra Comnenus. The Komnenos dynasty of Byzantine emperors was founded by Isaac I Komnenos, a Stratopedarch of the East under Michael VI. In 1057 Isaac led a coup against Michael and was proclaimed emperor. However, the dynasty did not come to full power until the accession of Alexios I Komnenos, Isaac I's nephew, in 1081. By this time, descendants of all the previous dynasties of Byzantium seem to have disappeared from the realm, such as the important Sclerus and Argyrus families. Descendants of those emperors lived abroad, having married into the royal families of Russia, France, Germany, Poland and Hungary; thus it was easier for the Komnenos family to ascend to the throne.
The Komnenoi were related to the Doukas family, whereby the clan often was referred as "Komnenodoukai" (or "Comnenoducai") and both surnames were used together by several individuals. Alexios I married Irene Doukaina, the grand-niece of Constantine X Doukas, a general who had succeeded Isaac I in 1059. Several families descended from the Komnenodoukai, such as Palaeologus, Angelos, Vatatzes and Laskaris. Alexios and Irene's youngest daughter Theodora made the future success of the Angelos family by marrying into it: Theodora's grandsons were the emperors Isaac II Angelos and Alexios III Angelos.
Under Alexios I and his successors the Empire was fairly prosperous and stable. Alexios moved the imperial palace to the Blachernae section of Constantinople. Much of Anatolia was recovered from the Seljuk Turks, who had captured it just prior to Alexios' reign. Alexios also saw the First Crusade pass through Byzantine territory, leading to the establishment of the Crusader states in the east. The Komnenos dynasty was very much involved in crusader affairs, and also intermarried with the reigning families of the Principality of Antioch and the Kingdom of Jerusalem - Theodora Komnene, niece of Manuel I Komnenos, married Baldwin III of Jerusalem and Maria, grand-niece of Manuel, married Amalric I of Jerusalem.
Remarkably, Alexios ruled for 37 years, and his son John II ruled for 25, after uncovering a conspiracy against him by his sister, the chronicler Anna Komnene, and her husband Nicephorus Bryennios. John's son Manuel ruled for another 37 years.
The Komnenos dynasty produced a number of branches. As imperial succession was not in a determined order but rather depended on personal power and the wishes of one's predecessor, within a few generations several relatives were able to present themselves as claimants. After Manuel I's reign the Komnenos dynasty fell into conspiracies and plots like many of their ancestors (and the various contenders within the family sought power and often succeeded in overthrowing the preceding kinsman); Alexios II, the first Komnenos to ascend as a minor, ruled for three years and his conqueror and successor Andronikos I ruled for two, overthrown by the Angelos family under Isaac II who was dethroned and blinded by his own brother Alexios III. The Angeloi were overthrown during the Fourth Crusade in 1204, by a relative from the Doukas family.
After the collapse of the empire in 1204, a branch of the Komnenoi fled back to their homeland in Paphlagonia, and set up the Empire of Trebizond on the Black Sea. The first emperor, also named Alexios I, was the grandson of Andronikos I. These emperors, the "Grand Komnenoi" (Megai Komnenoi) as they were known, ruled in Trebizond for over 250 years, until David Komnenos was defeated and executed by the Ottoman sultan Mehmed II.
A prince of the Komnenos family, great-grandson of Alexios I, Michael Angelos Komnenos Ducas (Michael I Ducas) founded in 1204 the Despotate of Epirus, following the dissolution of the Byzantine Empire after the Fourth Crusade.
A renegade member of the Komnenos family, also named Isaac, established a separate "empire" on Cyprus in the 12th century. The island was taken from him by Richard I of England during the Third Crusade.