In 1845-1848, the movement from the Lutheran Church to the Russian Orthodox Church took place in all the southern Estonian counties and about 17% of the peasants in southern Estonia converted to Orthodoxy. Until then, Orthodoxy was mainly the religion of the local Russians and Seto (Setu) people, and remained influential among the poluverniks of eastern Estonia, the Russians who were officially Lutheran but followed many Orthodox rites (including partially Estonianised Russians). The article gives an overview of the spread of Orthodoxy in the current Estonian territory and in Setomaa from the 11th century until 1845, focusing on the establishment of different Russian Orthodox churches and chapels (including the Seto tsassons). The Russian Old Believers, who settled in Estonia at the end of the 17th century are not dealt with in detail in this article. Orthodoxy is probably the most ancient form of Christianity to arrive in Estonia, during the 11th century. Some of the local Finno-Ugric people were baptised into Orthodoxy during the 11th-12th centuries, before the crusades of the Roman Catholic Church; it is also possible that the first Christian church in Estonia was founded by the Russian conquerors in Tartu (Yuryev) in the 11th century. The oldest surviving, although extensively reconstructed, Orthodox churches are to be found in Setomaa, and they date back to the 14th century. The oldest wooden sacral buildings in mainland Estonia are the Mikitamae and Uusvada tsassons (Seto village chapels), built in the last decade of the 17th century. The Orthodox sacral buildings also include the oldest surviving wooden church in Tallinn - the Kazan Church (1721). By the end of the Swedish period, the church of St. Nikolay (St. Nicholas) in Tallinn was the only active Orthodox church in Estonia (excluding Setomaa), but the gatherings around the Orthodox chapels in present-day East Viru County continued during the reign of Lutheran Sweden, especially crowded meetings were held around the Puhtitsa chapel. After the Great Northern War and incorporation into Russia, new Orthodox churches were erected in all the bigger towns in Estonia (first in 1721), as well as in many smaller places in eastern Estonia (e.g. Rapina, Nina, Mustvee and Vasknarva). Until the 1840s, the Orthodox churches were mostly built for Russians. However, many Estonians had had contacts with Orthodoxy for centuries before the 1840s, particularly in eastern Estonia and in some bigger towns.