The themes or thémata (Greek: θέματα, thémata, singular: θέμα, théma) were the main military and administrative divisions of the middle Byzantine Empire. They were established in the mid-7th century in the aftermath of the Slavic migrations to the Balkans and Muslim conquests of parts of Byzantine territory, and replaced the earlier provincial system established by Diocletian and Constantine the Great. In their origin, the first themes were created from the areas of encampment of the field armies of the East Roman army, and their names corresponded to the military units that had existed in those areas. The theme system reached its apogee in the 9th and 10th centuries, as older themes were split up and the conquest of territory resulted in the creation of new ones. The original theme system underwent significant changes in the 11th and 12th centuries, but the term remained in use as a provincial and financial circumscription until the very end of the Empire.

During the late 6th and early 7th centuries, the Byzantine Empire was under frequent attack from all sides. The Sassanid Empire was pressing from the east on Syria, Egypt, and Anatolia. Slavs and Avars raided Thrace, Macedonia, Illyricum, and southern Greece and settled in the Balkans. The Lombards occupied northern Italy, largely unopposed. In order to face the mounting pressure, in the more distant provinces of the West, recently regained by Justinian I (r. 527–565), Emperor Maurice (r. 582–602) combined supreme civil and military authority in the person of an exarch, a viceroy, forming the exarchates of Ravenna and Africa. These developments overturned the strict division of civil and military offices, which had been one of the cornerstones of the reforms of Diocletian (r. 284–305). Said administrative restructurings also found a precedent in Justinian's broad reorganization in the western conquests, denoting combined powers to the newly established Praetorian prefects of Africa (Eparchos tes Afrikís) and Italy (Eparchos tes Italías) respectively.

Justinian also endowed governors (eparchs, stratelates) of the eastern provinces plagued by brigandage and foreign invasions with military and administrative powers, formally abolishing the empire's dioceses, Diocletian's main administrative structure, but more importantly, he had also created the exceptional combined military-civilian circumscription of the quaestura exercitus and following the norm, abolished the Diocese of Egypt putting a dux (Greek: stratelates) with combined authority at the head of each of its old provinces instead. The empire maintained this precedent structure until the 640s, when the eastern part of the Empire faced the onslaught of the Muslim Caliphate. The rapid Muslim conquest of Syria and Egypt and consequent Byzantine losses in manpower and territory meant that the Empire found itself struggling for survival.

In order to respond to this unprecedented crisis, the Empire was drastically reorganized. As established by Hellenistic political practice, philosophies and Orthodox doctrines, power had been concentrated in military leaders strategoi who acted as viceroys in their respective "théma", being appointed by the emperor alone. Their main function around each was the collection of taxes from the different communities "chora", "komai" and from the different states "proasteion" as well as the management of fast and flexible provincial armies. The remaining imperial territory in Asia Minor was divided into four large themes, and although some elements of the earlier civil administration survived, they were subordinated to the governing general or stratēgos.

The origin and early nature of the themes has been heavily disputed amongst scholars. The very name théma is of uncertain etymology, but most scholars follow Constantine Porphyrogennetos, who records that it originates from Greek thesis ("placement"). The date of their creation is also uncertain. For most of the 20th century, the establishment of the themes was attributed to the Emperor Heraclius (r. 610–641), during the last of the Byzantine–Sassanid Wars. Most notable amongst the supporters of this thesis was George Ostrogorsky who based this opinion on an extract from the chronicle of Theophanes the Confessor mentioning the arrival of Heraclius "in the lands of the themes" for the year 622. According to Ostrogorsky, this "shows that the process of establishing troops (themes) in specific areas of Asia Minor has already begun at this time." This view has been objected to by other historians however, and more recent scholarship dates their creation later, to the period from the 640s to the 660s, under Constans II (r. 641–668). It has further been shown that, contrary to Ostrogorsky's conception of the thémata being established from the outset as distinct, well-defined regions where a stratēgos held joint military and civil authority, the term théma originally seems to have referred exclusively to the armies themselves, and only in the later 7th or early 8th centuries did it come to be transferred to the districts where these armies were encamped as well.

Tied to the question of chronology is also the issue of a corresponding social and military transformation. The traditional view, championed by Ostrogorsky, holds that the establishment of the themes also meant the creation of a new type of army. In his view, instead of the old force, heavily reliant on foreign mercenaries, the new Byzantine army was based on native farmer-soldiers living on state-leased military estates (compare the organization of the Sasanian aswārān). More recent scholars however have posited that the formation of the themes did not constitute a radical break with the past, but rather a logical extension of pre-existing, 6th-century trends, and that its direct social impact was minimal.

What is clear is that at some point in the mid-7th century, probably in the late 630s and 640s, the Empire's field armies were withdrawn to Anatolia, the last major contiguous territory remaining to the Empire, and assigned to the districts that became known as the themes. Territorially, each of the new themes encompassed several of the older provinces, and with a few exceptions, seems to have followed the old provincial boundaries. The first four themes were those of the Armeniacs, Anatolics and Thracesians, and the Opsician theme. The Armeniac Theme (Θέμα Ἀρμενιακῶν, Théma Armeniakōn), first mentioned in 667, was the successor of the Army of Armenia. It occupied the old areas of the Pontus, Armenia Minor and northern Cappadocia, with its capital at Amasea. The Anatolic Theme (Θέμα Ἀνατολικῶν, Anatolikōn), first mentioned in 669, was the successor of the Army of the East (Aνατολῆ, Anatolē). It covered southern central Asia Minor, and its capital was Amorium. Together, these two themes formed the first tier of defence of Byzantine Anatolia, bordering Muslim Armenia and Syria respectively. The Thracesian Theme (Θέμα Θρᾳκησίων, Théma Thrakēsiōn), first mentioned clearly as late as c. 740, was the successor of the Army of Thrace, and covered the central western coast of Asia Minor (Ionia, Lydia and Caria), with its capital most likely at Chonae. The Opsician Theme (Θέμα Ὀψικίου, Théma Opsikiou), first mentioned in 680, was constituted from the imperial retinue (in Latin Obsequium). It covered northwestern Asia Minor (Bithynia, Paphlagonia and parts of Galatia), and was based at Nicaea. Uniquely, its commander retained his title of kómēs (κόμης, "count").

In addition, the great naval division of the Carabisians or Karabisianoi (Kαραβισιάνοι, "people of the κᾱ́ρᾰβοι [ships]"), first mentioned in 680, was probably formed of the remains of the Army of the Illyricum or, more likely, the old quaestura exercitus. It never formed a theme proper, but occupied parts of the southern coast of Asia Minor and the Aegean Islands, with its stratēgos seat most likely at Samos. It provided the bulk of the Byzantine navy facing the new Arab fleets, which after the Battle of the Masts contested control of the Mediterranean with the Empire. In the event, the Carabisians would prove unsatisfactory in that role, and by 720 they had been disbanded in favour of a fully fledged naval theme, that of the Cibyrrhaeots (Θέμα Κιβυρραιωτῶν, Thema Kibyrrhaiotōn), which encompassed the southern coasts of Asia Minor and the Aegean islands.