During the early stages of the Scandinavian Viking Age, Ystad in the Danish province Scania and Paviken on Gotland were flourishing centers of trade, but they were not parts of the early Swedish Kingdom. Remains of what is believed to have been a large market dating from 600 to 700 CE have been found in Ystad. In Paviken, an important center of trade in the Baltic region during the 9th and 10th century, remains have been found of a large Viking Age harbor with shipbuilding yards and handicraft industries. Between 800 and 1000, trade brought an abundance of silver to Gotland, and according to some scholars, the Gotlanders of this era hoarded more silver than the rest of the population of Scandinavia combined.
St Ansgar is usually credited with introducing Christianity in 829, but the new religion did not begin to fully replace paganism until the 12th century. During the 11th century, Christianity became the prevalent religion, and from 1050 Sweden is counted as a Christian nation. The period between 1100 and 1400 was characterized by internal power struggles and competition among the Nordic kingdoms. In the years 1150–1293 according to the legend of Eric IX and the Eric Chronicles Swedish kings made a first, second and third crusade to pagan Finland against Finns, Tavastians and Karelians and started conflicts with the Rus’ who no longer had any connection with Sweden. The Swedish colonization of the coastal areas of Finland started also during the 12th and 13th century. In the 14th century, the Swedish colonization of coastal areas of Finland began to be more organized and in the end of the century several of the coastal areas of Finland were inhabited mostly by Swedes.
Except for the provinces of Scania, Blekinge and Halland in the south-west of the Scandinavian peninsula, which were parts of the Kingdom of Denmark during this time, feudalism never developed in Sweden as it did in the rest of Europe. The peasantry, therefore, remained largely a class of free farmers throughout most of Swedish history. Slavery (also called thralldom) was not common in Sweden, and what slavery there was tended to be driven out of existence thanks to the spread of Christianity as well as to the difficulty to obtain slaves from the lands east of the Baltic Sea, and by the development of cities before the 16th century. Indeed, both slavery and serfdom were abolished altogether by a decree of King Magnus IV in 1335. Former slaves tended to be absorbed into the peasantry, and some became laborers in the towns. Still, Sweden remained a poor and economically backward country in which barter was the primary means of exchange. For instance, the farmers of the province of Dalsland would transport their butter to the mining districts of Sweden and exchange it there for iron, which they would then take to the coast and trade for fish, which they consumed, while the iron would be shipped abroad.
In the middle of the 14th century, Sweden was struck by the Black Death. The population of Sweden and most of Europe was seriously decimated. The population did not reach the numbers of the year 1348 again until the beginning of the 19th century. One third of the population died in the triennium of 1349–1351. During this period, the Swedish cities began to acquire greater rights and were strongly influenced by German merchants of the Hanseatic League, active especially at Visby. In 1319, Sweden and Norway were united under King Magnus Eriksson, and in 1397 Queen Margaret I of Denmark effected the personal union of Sweden, Norway, and Denmark through the Kalmar Union. However, Margaret’s successors, whose rule was also centered in Denmark, were unable to control the Swedish nobility.