The themes or themata (Greek θέματα; singular θέμα thema) of the Byzantine Empire were administrative units established by a reform promulgated by Emperor Heraclius in the 7th century.

Reasons for Heraclian reforms
The new system of settling military units in vacant lands and thus strengthening local loyalties to the state greatly helped the Byzantine Empire. Over the next several decades, the Sassanids retreated, the Slavs and Avars were reduced, and rebellions became far less common. The new military structure rescued the Empire from destruction and gave it a durability that would last for centuries. However, there was a price to be paid, in terms of a militarization of society and a decline of civil institutions and civil culture; for this reason, the introduction of the themes is often seen as marking the end of Late Antiquity and the beginning of the Middle Ages for the Eastern Roman Empire.
The theme system, in time, created aristocratic families such as the Phocades, deeply entrenched in their local area, and deploying what amounted to private armies. These families, having troops loyal to them instead of the emperor and being financially autonomous, often challenged or even usurped imperial authority.

Organization of themata
Each of the original five themata was formed from the Empire's earlier mobile field armies. As the empire had shrunk, most of the armies had retreated to newer stations in the interior. Heraclius assigned each mobile army a part of Anatolia. Because the language of the empire was also being changed from Latin to Greek, the themes acquired Hellenized names.
The Opsician theme was formed from the armies in the Emperor's presence, which had lately been known as the Obsequium (retinue). The armies in the Emperor's presence had been stationed in southern Thrace and northwestern Anatolia, near the capital of Constantinople, and this was where the Opsician Theme was formed.
The Army of Armenia became the Armeniac theme, stationed in most of its original territory in eastern Anatolia, to the west of the Armenian protectorate. The Army of the East, which had formerly defended Roman Syria and Palestine, retreated when those areas were lost first to the Persians and later to the Arabs. They were settled in central Anatolia and became the Anatolic theme. The Army of Thrace became the Thracesian theme, settled in western Anatolia where Heraclius had withdrawn it. Emperor Constans also created a corps of marines, the Carabisian theme, named after a Greek word for ship (karabis) and based in Greece, in the Aegean islands and on the southern shore of Anatolia. This appears to have been formed from the remains of the Army of Illyricum, whose territory had included Greece. Early in the eleventh century, the Byzantine annexation of several Georgian and Armenian lands resulted in the creation of the theme of Iberia, which eventually collapsed under the Seljuk attacks in the 1060-70s.