As in other parts of the Roman world, the function of cities in the administrative structure of the state underwent a gradual evolution from the 3rd century on as the central government found it increasingly necessary to intervene in municipal affairs in order to gain revenues. This need for intervention developed when the vitality of cities became eroded as a result of a range of factors, notably the economic damage to urban infrastructures that accompanied the civil wars and barbarian inroads of the 3rd century. The so-called “decline” of the curial order (the strata of local government officials known as decurions), caused by the weakening of the fiscal and economic independence of many towns by the tendency of urban elites to avoid municipal obligations, also played a role. Finally, in the eastern parts of the empire, the transformation of Byzantium into Constantinople as a new imperial capital in 330 had important results for patterns of internal trade and commerce as well as for social relations between provincial elites and the state. Nevertheless, the cities of the southern Balkans were able to survive the raids and devastation of both Goths and Huns in the 4th and 5th centuries, and there is no evidence that cities ceased to carry on their function as centres of market activity, local administration, and social life. Those cities that were artificial, or purely administrative creations of the Roman state, were the first to suffer, and, under difficult conditions, they generally disappeared. On the other hand, the state—through its regional and central military administration—also appears to have been involved in promoting different types of smaller defended urban centres, which were better adapted to these conditions.

In the 7th century a series of developments combined to further promote what was already in the process of becoming a radically different pattern of settlement and administration. In the first place, the results of the long-term developments already referred to, in terms of both social and economic stability, were factors. In particular, the municipal landowning elites transferred their attention—especially with respect to investment in the imperial system—away from their local cities to the imperial capital. In the second place, the economic disruption brought about by the infiltration of large numbers of Slav settlers and immigrants was exacerbated by the devastation and insecurity caused by the wars between the empire and the Avars, a Turco-Mongol confederacy that was able, from the 560s to the 630s, to harness the resources of those peoples in the plains of Hungary and in the Balkans to launch a series of extremely damaging and disruptive attacks on the Eastern Empire. Although the disruptions of the earlier invasions of the Huns and the Goths may have been as damaging in the short term, the Balkans were by this time suffering from the cumulative effects both of two centuries of constant insecurity and warfare and of endemic plague, which had struck the Eastern Empire in the 540s. It is clear from the imperial legislation of the late 5th and early 6th centuries dealing with the Danubian provinces and with Thrace, as well as from the archaeological record, that the final results of this economic disruption were to bring about the abandonment of the traditional pattern of urban economies. Even the construction of churches, which had experienced a certain efflorescence during the 6th century, virtually ceased in the 7th century. Secular construction also stopped, an extended process of ruralization of settlement and of economic life took place, and new populations penetrated far into the Peloponnese. The extent to which these new settlers replaced, or were able to live alongside, the indigenous population remains unclear.

As a consequence of these changes, the traditional administration collapsed, chiefly because the government at Constantinople could control only the coastal plains and some river valleys. The cities that remained in imperial hands, such as Corinth (Kórinthos) and Thessalonica (Thessaloníki), for example, were well-defended fortresses or had access to the sea, whence they could be supplied. Inland, tribal groupings and chieftaincies (sklaviniai) dominated many districts, and, though it appears that the empire claimed political sovereignty over such regions, it was rarely able to make this effective or to extract regular revenues, although tributes were obtained at times. The sklaviniai, however, did not control the entire inland region; there were many areas in which the indigenous population and the traditional patterns of local social structure and political organization may have survived and where imperial authority may have been recognized. The evidence is too sparse to draw definite conclusions.

Evidence for the degree of Byzantine control over the area is reflected dimly in the lists of signatories to ecclesiastical councils (especially those of 680 at Constantinople and of 787 at Nicaea [modern İznik, Turkey]) and in the various lists of bishoprics (Notitiae episcopatuum). From these it is clear that ecclesiastical administration in the south, especially in the Peloponnese, had suffered considerably. Many bishoprics were abandoned and had ceased to exist. At the council of 680, only 4 bishops from that region (Athens, Corinth, Lakedaimon, and Árgos) and 12 from Macedonia attended; what is believed to be a 7th-century episcopal list (the Pseudo-Epiphanios) records the names of only 5 metropolitan bishops from Greece, mostly from the north.

Administratively, those districts that remained under Byzantine control were organized from the later 7th century onward into the military province, or theme (Greek: thema), of Hellas, under its general (strategos). The theme initially encompassed only the easternmost parts of central Greece but gradually included parts of Thessaly and, possibly, of the Peloponnese, although in the latter case only the coastal regions were involved.

The islands of the Aegean remained largely in Byzantine hands. In late antiquity they had been relatively heavily populated, the larger ones among them—especially Lemnos (Límnos) and Thasos (Thásos) in the north—being well-known sources of agricultural produce. Arab piracy and raiding from the later 7th century onward altered this, causing many of the smaller islands to become deserted; however, the islands recovered during the 10th century.

Byzantine fortress-towns testify to the presence of sizable rural populations needing shelter from attack; the towns also served as refuges for people from the mainland fleeing Avar, Slav, or Bulgar raids. Administratively, in the second half of the 7th century they formed a component of the districts allotted to the naval theme (in the original sense of the term—an “army”) of the Karabisianoi (Greek: karabos, a light ship), which represented the rump of the quaestura exercitus (the Roman military command comprising the Islands, Caria, and Moesia II). During the first half of the 8th century this command appears to have been subdivided to form the naval themes of the Kibyrrhaiotai (including parts of western Asia Minor and named after the district and town of Kibyrrha in southwestern Asia Minor), Samos (Sámos), or the Kólpos (“Gulf”) in the southern zone, and Aigaion Pelagos covering the northern districts. Further subdivisions took place in the 10th–12th century, so that commands of the Dodecanese (Dodekánisa) Islands or the Cyclades (Kykládes) also appear in the sources.

It must be stressed that, even though the archaeological record clearly supports the conclusion that a dramatic collapse of traditional urban society and economic relations occurred, it also gives evidence of significant regional variations. The process of change was neither uniform across Greece nor was it always as extreme in one area as in another. The degree of access to imperial resources and the ability to maintain regular contact with the imperial government were factors that gave some areas a very different appearance from others. It is important that this be borne in mind when considering the history of the various regions of Greece in the following period.