Boris I of Bulgaria in the context of "Knyaz"

The Christianization of Bulgaria was the process by which 9th-century medieval Bulgaria converted to Christianity. It reflected the need of unity within the religiously divided Bulgarian state as well as the need for equal acceptance on the international stage in Christian Europe. This process was characterized by the shifting political alliances of Boris I of Bulgaria (ruled 852–889) with the kingdom of the East Franks and with the Byzantine Empire, as well as his diplomatic correspondence with the Pope.

Because of Bulgaria's strategic position, the churches of both Rome and Constantinople each wanted Bulgaria in their sphere of influence. They regarded Christianization as a means of integrating Slavs into their region. After some overtures to each side, the Khan adopted Christianity from Constantinople in 870. As a result, he achieved his goal of gaining an independent Bulgarian national church and having an archbishop appointed to head it.

Tzeésthlabos (Greek: Τζεέσθλαβος), usually transliterated as Časlav (Serbian Cyrillic: Часлав) or Chaslav and Tzeeslav, was Prince of the Serbs from c. 933 until his death in c. 943/960.

Časlav was the son of Klonimir, a son of Strojimir who ruled as co-prince in 851–880. He belongs to the first Serbian dynasty, the Vlastimirovićs (ruling since the early 7th century), and is the last known ruler of the family. His mother was a Bulgarian noblewoman chosen as wife of Klonimir by Boris I of Bulgaria himself.

The Preslav Literary School (Bulgarian: Преславска книжовна школа), also known as the "Pliska Literary School" or "Pliska-Preslav Literary school" was the first literary school in the medieval First Bulgarian Empire. It was established by Boris I in 886 in Bulgaria's capital, Pliska. In 893, Simeon I moved the seat of the school from the First Bulgarian capital Pliska to the new capital, Veliki Preslav. Preslav was captured and burnt by the Byzantine Emperor John I Tzimisces in 972 in the aftermath of Sviatoslav's invasion of Bulgaria.

The monarchs of Bulgaria ruled Bulgaria during the medieval First (c. 681–1018) and Second (1185–1422) Bulgarian empires, as well as during the modern Principality (1879–1908) and Kingdom (1908–1946) of Bulgaria. This list includes monarchs from the establishment of the First Bulgarian Empire until modern times, omitting earlier mythical rulers as well as rulers of separate states such as Old Great Bulgaria and Volga Bulgaria.

Various titles have been used by the rulers of Bulgaria. The only recorded title, used before Bulgaria's conversion to Christianity, is kanasubigi, likely meaning "Khan, Lord of the Army" or "the sublime Khan". When Bulgaria converted to Christianity in the ninth century, the ruler Boris I (852–889) was using the title knyaz (prince). For much of its later history under the first and second empires, Bulgaria functioned as a multi-ethnic imperial state modelled on the neighbouring Byzantine Empire, which contributed to the adoption of the title of tsar (emperor) by Bulgarian monarchs beginning with Simeon I (893–927) in 913. Some powerful medieval Bulgarian rulers challenged Byzantine authority by proclaiming themselves as both Bulgarian and Roman emperors.