The settlement of Porsgrunn started in the 1650's as a deep-water harbor for Skien and was considered part of Skien until 1842. The name "Porsgrunn" comes from the "pors"  (bog myrtle) shrub that still grows around Eidanger. The kommune's emblem has the city's small river represented by a diagonal silver line.  The silver anchor on blue background represents the importance of the harbor, and the branch on the red background is from the bog myrtle shrub.

Porsgrunn kommune includes the town of Brevik and the Eidanger area. Although sea coast, forests, lakes, and farm land lie within the city limits, the city has a strong concentration of industrial activity. Norsk Hydro's Porsgrunn factories at Herøya are Norway's largest single place of employment. Other important industries are Norcem Company, Elkem, Porsgrunds Porselønsfabrik, Isola, and Beha. The neighborhood of Osebakken contains well-preserved homes of wealthy sea captains.

Langangen at the head of the Langangen fjord is most famous for the "Corkscrew," a curlicue highway now replaced by two high suspension bridges. Bergsbygda, south of highway E 18, has Stone Age sites and a hill fort that bear witness to ancient settlement. Hollander House, a wooden structure from the 1600's, is a remembrance of the international trade relations brought about by the flourishing timber exports of the past.

Brevik was an independent community until its union with Porsgrunn in 1964 and an important export harbor for timber and ice. It became an official town by royal decree in 1845 but was a center of commerce as early as the 16th century. It is located on the Eidanger headland. Many of the original wooden buildings have been preserved and are still in use. On Sylte Island you will find the almost intact old neighborhood of captains' and seamans' houses. Early in the summer "Brevik Days" are celebrated with concerts, exhibitions, guided tours through the old neighborhoods, and other activities. The naval hero admiral Curt Sivertsen Adeler, 1622-1675, came from Brevik. Currently the Dalen Portland cement company is the largest employer.