From the late 11th century on, during the Komnenian period, it and variants derived from it formed the basis of a new system of court titles for the Byzantine Empire. The female form of the title was sebastē (Greek: σεβαστή). It also formed the basis for the creation of the title of sebastokratōr.
The term was used in the Hellenistic East as an honorific for the Roman emperors from the 1st century onwards. For example, the Temple of the Sebastoi in Ephesus is dedicated to the Flavian dynasty. This association also was carried over to the naming of cities in honor of the Roman emperors, such as Sebaste, Sebasteia and Sebastopolis. Following the adoption of the term basileus as the main imperial title in the 7th century, the epithet fell out of use, but it was revived in the mid-11th century by Emperor Constantine IX Monomachos (reigned 1042–1055) for his mistress Maria Skleraina. Thereafter, the title began to be conferred upon members of the nobility favored by the Byzantine emperors, including Bagrat IV of Georgia, George II of Georgia, and the future emperor Alexios I Komnenos (r. 1081–1118). When the latter assumed the Byzantine throne in 1081, he set about to reorganize the old system of court dignities, with the sebastos as the basis for a new et of titles, which primarily signalled the closeness of their holders' familial relationship to the emperor. This use of the imperial epithet of sebastos set the imperial family apart, at the top of the imperial hierarchy, and made them, in the words of historian Paul Magdalino, "partners in, rather than executives of, imperial authority." In this context, the scholar L. Stiernon calculated that over 90 percent of the sebastoi belonged to the ruling Komnenos family.
In 12th-century Byzantium, the sebastoi were divided in two groups: the simple sebastoi and the sebastoi gambroi. The latter were members of various aristocratic families tied to the emperor via marriage to his female relatives (gambros means "son-in-law" in Greek). The gambroi thus formed the upper layer of the sebastoi class. The title was also conferred to foreign rulers, and spread to neighboring, Byzantine-influenced states, like Bulgaria, where a sebastos was the head of an administrative district, and Serbia, where the title was employed for various officials. In Byzantium itself, the title lost its pre-eminence in the late 12th century, and in the following centuries the sebastos was a title reserved for commanders of ethnic units.
The title of prōtosebastos (Greek: πρωτοσέβαστος, "first sebastos") was probably created by Emperor Alexios I Komnenos, and first conferred to his brother Adrianos. It was also conferred on Sergius VI of Naples and his son, John VI, at about the same time. Later, during the 12th century, it was given to close relatives of the Byzantine emperor, such as the sons of a sebastokratōr. The title remained relatively important during the Palaiologan period as well, being listed by pseudo-Kodinos as coming after the megas logothetes and before the pinkernes ("cupbearer").
The title of panhypersebastos (Greek: πανυπερσέβαστος, "venerable above all") was also created by Alexios I, and conferred to members of aristocratic families closely allied to the imperial family. Michael Taronites, Alexios's brother-in-law, was first awarded this title and regarded as almost equal to a Caesar. The title remained very important through to the Palaiologan era, following the megas domestikos, the overall army commander. Perhaps the most notable of its holders was John VI Kantakouzenos, who held the title before his proclamation as Byzantine emperor in 1341.
- Kazhdan, Alexander Petrovich, ed. (1991). The Oxford Dictionary of Byzantium. New York, New York and Oxford, United Kingdom: Oxford University Press. ISBN 978-0-19-504652-6.
- Magdalino, Paul (2002) . The Empire of Manuel I Komnenos, 1143–1180. Cambridge, United Kingdom: Cambridge University Press. ISBN 0-521-52653-1.
- Stiernon, Lucien (1965). "Notes de titulature et de prosopographie byzantines: Sébaste et gambros". Revue des études byzantines (23).
- von Falkenhausen, Vera (2007). "The South Italian Sources". Proceedings of the British Academy 132.