By the 13th century, the following major counties had developed in Estonia: Saaremaa (Oesel), Läänemaa (Rotalia or Maritima), Harjumaa (Harria), Rävala (Revalia), Virumaa (Vironia), Järvamaa (Jervia), Sakala (Saccala), and Ugandi (Ugaunia).

The name Eysysla appears sometimes together with Adalsysla, 'the big land', perhaps 'Suuremaa' or 'Suur Maa' in Estonian which refers to mainland Estonia.

Saaremaa was the wealthiest county of ancient Estonia and the home of notorious Estonian pirates, sometimes called the Eastern Vikings.

The Chronicle of Henry of Livonia describes a fleet of sixteen ships and five hundred Oeselians ravaging the area that is now southern Sweden, then belonging to Denmark.

The Oeselians along with the Curonians were known in the Old Norse Icelandic Sagas and in Heimskringla as Víkingr frá Esthland (in English, Estonian Vikings).

[13] On the eve of the Northern Crusades, the Oeselians were summarized in the Livonian Rhymed Chronicle thus: "The Oselians, neighbors to the Kurs (Curonians), are surrounded by the sea and never fear strong armies as their strength is in their ships.

"[14] In 1206, the Danish army led by king Valdemar II and Andreas, the Bishop of Lund landed on Saaremaa and attempted to establish a stronghold, but without success.

In 1216 the Livonian Brothers of the Sword and the bishop Theodorich joined forces and invaded Saaremaa over the frozen sea.

In 1220, the Swedish army led by king John I of Sweden and the bishop Karl of Linköping conquered Lihula in Rotalia in Western Estonia.

In 1222, Valdemar II again tried to conquer Saaremaa, this time establishing a stone fortress housing a strong garrison.

The Oeselians again accepted Christianity by signing treaties with the Master of Teutonic Order in Livonia Andreas de Velven and the Bishopric of Ösel-Wiek in 1241.

The next treaty was signed in 1255 by the Master of the Order, Anno Sangerhausenn, and, on behalf of the Oeselians, by men whose "names" (or declaration) were transcribed by Latin scribes as Ylle, Culle, Enu, Muntelene, Tappete, Yalde, Melete, and Cake[16] The treaty granted several distinctive rights to the Oeselians.