Porsgrunn has been an important harbor town in the Grenland area since the late 16th century. In 1653, the Customs House was moved further down the Telemarksvassdraget from Skien to Porsgrunn mainly because industrial waste such as sawdust andmud made the river too shallow to allow boats to go any further up the river. Moving the Custom House from Skien to Porsgrunn in 1653, added to the flourishing harbor activity and Porsgrunn became a thriving market town.
In the 18th century, it was the home of some of Norway's most influential families at the time, such as the Aalls, Cappelens,L�venskiolds, and Deichmans. Also in this period, Porsgrunn was considered the cultural centre of Norway. On the ecclesiastical side, Porsgrunn was separated from the ancient rural parishes of Eidanger, Solum, and Gjerpen in 1764 to become a prestegjeld with its own minister. It was granted limited city status in 1807, but this was expanded to full city status in 1842
