Mount Athos as a monastic community was formally founded in 963, when the monk Athanasios established the monastery of Great Lavra, still the largest and most prominent of the 20 monasteries. It enjoyed the protection of the emperors of the Byzantine Empire during the following centuries and its wealth and possessions grew considerably. The Fourth Crusade in the 13th century brought new Roman Catholic overlords which forced the monks to seek protection from Pope Innocent III, until the restoration of the Byzantine Empire. It was raided by Catalan mercenaries in the 14th century, a century that also saw the theological conflict over the hesychasm practised on Mount Athos and defended by Gregory Palamas.
The Byzantine Empire collapsed in the 15th century and the newly established islamic Ottoman Empire took over. They heavily taxed the monasteries, but for the most part left them alone. The population of monks and their wealth declined over the next centuries, but was revitalised around the 19th century by the donations and new arrivals from other Eastern Orthodox countries, such as Russia, Bulgaria, Romania and Serbia and each country came to exert its influence on individual monasteries. In 1912, during the First Balkan War, the Ottomans were forced out and after a brief conflict between Greece and Russia over sovereignty, the pensinsula formally came under Greek sovereignty after World War I.
Politically the peninsula is mostly self-governed and consists of 20 main monasteries and the capital city and administrative centre, Karyes, also home to a governor as the representative of the Greek state. Beyond the monasteries there are 12 sketae, smaller communities of monks, as well as many (solitary) hermitages throughout the peninsula. Visits to the peninsula are possible for laymen, but they need special permission.
Women are completely barred from the peninsula, a fact which has earned a certain amount of fame; even female domestic animals (with the exception, some say, of cats) are forbidden. However, during the Greek Civil War[?], Athos did shelter refugees including women and girls.  (http://www.straightdope.com/columns/010209)
Mount Athos: The Holy Mountain (http://www.macedonian-heritage.gr/Athos/intro) - History, culture, geography and visiting information