Eidanger was established as a municipality January 1, 1838 (see formannskapsdistrikt). It was merged with Porsgrunn January 1, 1964. The main part of Eidanger is a peninsula between the Eidangerfjord and the Frierfjord. Situated between the urban communities of Brevik and Porsgrunn, and with excellent natural conditions for building harbours, it became the home of major industrial sites as Norsk Hydro's plant at Herøya, and the Dalen Portland (now part of the Norcem corporation) concrete factory just outside of Brevik. Another major industry is Heistad Fabrikker, which makes products for diabetics. Another factory in Eidanger is Isola fabrikker and is the only factory of its kind in the world.