Mount Athos, also called the Holy Mountain (Hagion Oros), for the late 10th center is the most important center of Eastern Orthodox monasticism. Athos is the name given to the northernmost projection of the Chalkidike peninsula, 45 km long, 5-10 km wide, as well as to the peak (2,033 m) that dominates this rocky finger of land. It is linked to the mainland by a narrow isthmus 2 km in width. The peninsula has forests, meadows for pasturage, and small plots of land suitable for vineyards, orchards, olive groves, and gardens. Athos was virtually deserted when monks first began to settle there, probably in the late 8th or early 9th century. 
According to the 10th century historian Genesios Athos was already a major monastic community in 843, but his evidence must be treated with caution. The theories that the earliest monks of Athos were refugees from the Arab conquests of the eastern provinces of Byzantine, or Iconodules fleeing the persecutions of the Iconoclast emperors, have now lost favor. The first arrivals seem to have come from nearby regions, and to have been attracted by the unsullied solitude of the peninsula. Monasticism developed slowly on the Holy Mountain, however, because of its isolation, its rugged terrain, and the danger from Arab pirates. The early monks lived as solitary hermits or in small groups; the pioneers on Athos included Peter the Athonite - a semi-legendary figure - and Euthymios the Younger, who arrived in 859. 
The first cenobitic monastery in the vicinity of Athos was Kolobou, founded near Hierissos sometime before 883. A fragmentary sigillion of Basil I is the earliest preserved imperial act concerning the Holy Mountain; it protected the Athonite monks from the intrusion of local shepherds. The date of the first appearance of cenobitic (“life in common”) monasticism on Athos proper is impossible to ascertain, but by the mid-10th century some koinobia (for example, Xeropotamou Monastery) are attested. In 963 Athanasios of Athos, with the support of Nikephoros Phokas, founded the Great Lavra, which would soon hold first place in the Athonitc hierarchy, a position it would maintain in perpetuity. By the end of the 10th century, many of the most important Athonite monasteries (for example Iveron, Hilander, Esphigmenou, Panteleimon, Vatopedi, Xenophontos and possible Zographou) had been founded; by 1001 46 monasteries were in existence.
Monks from non-Greek lands began to come to the Holy Mountain in the 10th century. The Georgian monastery of Iveron was established in 979/80, soon followed by the Italian monastery of the Amalfitans. Orthodox Armenians (Chalcedonians) were numerous at Esphigmenou. In the 12th century  the peninsula began to attract more Slavic monks: Panteleimon was taken over by monks from Rus, and Hilandar was restored as a Serbian monastery. In the 13th century Zographou came to be inhabited primarily by Bulgarian monks. The organization of Athos in the 10th century was relatively simple: the monks attended three annual assemblies at the Protaton in Karyes and elected a pituros who represented the community in its relations with ecclesiastical and secular authorities. By the end of the 10th century it seems that this assembly was replaced by an irregular "council" that attracted on the average 15 participants, but occasionally as many as 40. The larger monasteries became independent of the Protaton, with the hegoumenos of the Great Lavra acquiring a more prestigious position in the local hierarchy than the protos. 
In the 10th and 11th centuries Athos attracted considerable imperial attention. Romanos I Lekapenos initiated an annual stipend (roga) for the Athonite monks and ordered the demarcation of a frontier boundary, probably in 941/2. The rapid growth of the Lavra under the patronage of Nikephoros Phokas prompted the resentment of many Athonite monks, especially the anchorites who feared for their way of life. John I Tzimiskes' issuance of a typikon for Athos, the Tragos, between 970 and 972, attempted a compromise, recognizing the rights of hegoumenoi, kelliotai (the spiritual leaders of anachoretic groups), and solitary hermits to attend the assemblies at Karyes. Both Nikephoros II and John I envisaged Athos as a stronghold of "poor monasticism," but under Basil II some monasteries began to acquire lands beyond the boundaries of the Holy Mountain and were gradually transformed into great landowners. Cenobitism became predominant, to the detriment of hermitages. In the 11th-12th centuries new monasteries continued to be founded (Kastamonitou, Docheiariou, Koutloumousiou), and the older ones expanded their possessions. Economic activities on Athos increased, such as the sale of wood from Athonite forests and surplus agricultural products (fruits, vegetables, wine) cultivated on monastic estates. Many monasteries owned boats for the transport of these goods and the importation of necessary provisions; these boats often were granted exemptions from customs duties. Despite John I's prohibition of the presence of eunuchs, beardless youths, women, and even female animals on the peninsula, in the 11th century substantial groups of Vlach shepherds settled with their families on Athos and supplied the monks with dairy products. The "Vlach question" caused such a scandal that around 1100 Alexios I was forced to expel the herdsmen from Athos. 
Constantine IX Monomachos's chrysobull of 1045 sheds light on the administrative development of Athos. The independence of individual koinobia increased; Lavra, Vatopedi, and Iveron were the top-ranking monasteries, taking precedence over the central administration of the protos. The growth of landownership incited conflicts among monasteries over estates as well as clashes with local landowners, especially in Hierissos; with the Cumans who had settled in southern Macedonia; and with imperial functionaries. On the other hand, the patriarchate tried to establish its jurisdiction (at least partial) over Athos, which had been considered as subordinate only to the emperor. 
The fall of Constantinople to the Fourth Crusade in 1204 and the establishment of the Latin Empire led to a period of difficulty for Athos, as Macedonia was troubled by the Latin occupation, the rising power of the Bulgarians, and rivalry between the Empire of Nicaea and Epirus. Athos came under the rule of the Frankish Kingdom of Thessaloniki from 1204 to 1224, and the monasteries lost some of their properties outside the peninsula, which they sought to recover after the Greek reconquest of Constantinople in 1261. The reign of Michael VIII Palaiologos was, however, extremely unpopular on Athos, because of the persecution of monks who refused to accept the Union of Lyons of 1274.
In the early 14th century Athos suffered from the raids of the Catalan Grand Company, but then enjoyed a period of prosperity during which several new monasteries were founded ( Dionysiou, Pantokrator, Simopetra). Documents recording various privileges conferred by the emperors on Athonite monasteries (a practice which goes back to the 9th century) are especially copious from the first half of the 14th century. Whereas the privileges granted by the government in the 10th century were primarily Solemnia (stipends from the state treasury) and the chrysobulls of the 11th century mostly established monastic exkausseia (immunity from taxes), the documents of the 14th century were first of all donations of lands and paroikoi. 
The properties of Athos took the form of fields, vineyards, pastures, mills, fishponds, entire villages, urban rental properties, and workshops. These possessions were concentrated in Mace-donna (including Thessaloniki), especially on the Chalkidike peninsula and in the Strymon valley, but extended to Thrace, Thasos, Lemnos, Serbia, and Wallachia. The bulk of the acts of Athos concern these estates, and include praktika, charters of sale, exchange, and donation, in addition to imperial chrysobulls confirming the monasteries' titles to their property and guaranteeing fiscal immunity. All ranks of people, from humble peasant to emperor, were anxious to make pious donations to Athonite monasteries; in addition to the emperors at Constantinople, the benefactors of Athos included the Grand Kornnenoi of Trebizond, the rulers of Ser-bia and Bulgaria, and voivodes of Wallachia. 
In the 14th century Idiorrhythmic monasticism developed on Athos, and the koinobion declined. By mid-century Turkish pirates were attacking the peninsula, forcing some of the monks to flee to Paroria or to Meteora. The Ottoman threat led to government restriction on the growth of monastic properties and the confiscation of some Athonite estates in the second half of the 14th century. Thus, after the Turkish victory at Marica in 1371 half of the metochia belonging to Athos were transformed into pronoiai and transferred to soldiers. This policy was continued in the 15th century. After briefly occupying Athos in 1387 and from 1393 to 1403, the Ottomans established permanent control over the Holy Mountain in 1430. The Turks recognized the autonomy of Athos in return for the payment of annual tribute, but the monasteries lost their immunities and their estates in Thrace and Macedonia. 
Attitudes toward the intellectual life were varied. Kelliotai and hermits, who placed an emphasis on spirituality and asceticism, had little use for books. Many of the Athonite monks came from a rustic background and were illiterate. Nonetheless in the koinobia, founded on the Stoudite model, there was more emphasis on intellectual pursuits, especially from the 13th century onward. The monasteries amassed important collections of manuscripts, some produced in their own scriptoria (for example at Philothcou, Hilandar, and Iveron). Among Athonite monks could be found composers (John Koukouzelis), hagiographers (Joseph Kalothetos), theologians (Gregory Palamas), and ecclesiastical writers (Theoleptos of Philadelphia). With its international assemblage of monks, cultural interchange was inevitable: Hilandar, Zographou, Panteleemon, and Iveron became centers for the transmission of Byzantine religious literature to Serbia, Bulgaria, Russia, and Georgia, respectively. 
As the Holy Mountain par excellence from the 10th century onward, Athos attracted Byzantine monks for six centuries. Many holy men, whose custom it was to wander from one monastery or Holy Mountain to another, spent time on Athos before moving on, thus reducing the cultural isolation of the Athonite monasteries. Because of its geographical proximity, Thessaloniki, rather than Constantinople, had the closest links with the Holy Mountain. For some monks, like Palamas, a hegoumenate on Athos was the springboard to a bishopric. For others it might lead to the patriarchate of Constantinople as it did for Niphon, Kallistos, and Philotheos Kokkinos.
It was one of the wandering holy men, Gregory of Sinai, who introduced to Athos in the 14th century the "Jesus prayer," which was adopted by a small number of monks. From this new method of prayer developed a form of mystical spirituality, a renewed emphasis on Hesychasm that was championed by Palamas. After many vicissitudes Palamism spread all over the Byzantine world and was eventually declared Orthodox by the local council of Constantinople of 1351.Little survives of the 10th-12th century architecture of the Holy Mountain except for the principal churches of a few monasteries and portions of the perimeter walls. The earliest Athonite churches generally had an inscribed-cross plan with a central dome, triconch apse, a double narthex, and lateral chapels to the west. Instituted at the Lavra, this scheme was adopted at Iveron and Vatopedi and remained essentially unchanged until the double narthex was replaced by a unified rectangular space (sometimes called a lite) for singers at Hilandar. This scheme, in turn, was widely adopted, for example, at Koutloumousiou around 1400. The 14th century saw an expansion of the older monasteries, the addition of towers (pyrgoi) and other fortifications, and the creation of new institutions that tended to follow the established "Athonite type." Most of the chapels and living and service quarters now to be seen on Athos date from the 15th C. or later.

In the churches mosaic decoration survives only at Vatopedi and Xenophontos (now detached and kept in the "new katholikon"). The oldest preserved frescoes are at the kellion of Rhabdouchou, while the frescoes of 1312 survive at Vatopedi but are much overpainted. The well-preserved program at the Protaton is of similar date. Thereafter, however, with the exception of fragments in the monastery of St. Paul, almost no wall painting survives from the period between the mid-14th and the early 16th century. From the 10th century onward, Athonite monasteries received gifts of liturgical silver, crosses, textiles, sometimes richly covered books, and especially icons (of which the Lavra has 3,000, mostly post-Byzantine), which form the nuclei of their treasures today. A few objects are the donations of generous rulers and other patrons from the period before 1453 but, like the physical fabric of the monasteries, the vast majority of the treasures date well after the foundation of the institutions that now house them. It has been suggested that at least in the 14th century fresco painters came from Thessaloniki and possibly Constantinople. The name or epithet zographos of a 10th-century monk suggests, however, that some artists took up residence; a 14th-centurt workshop that made icon frames has also been hypothesized. Certainly masons were called in from the outside world in the 10th century. Many of the illuminated manuscripts in the monasteries' libraries reached Athos long after their creation elsewhere, just as many books with Athonite provenances are today to he found in libraries and museums outside the Holy Mountain.