By the local government reform of 1971 it became a municipality of unitary type and amalgamated with Fliseryd and Ålem in 1974.

The municipal arms show the ridge which the town centre with its church has been situated on since the medieval age.

The cross symbolizes the Knights Hospitallers, a monastic order that was built in the town in the fifteenth century and had a large influence on its development.

It is an etymological design showing the origin of the name of the municipality: Mönster comes from monastery while ås means ridge.

There are two rivers flowing through the municipality towards to sea: Emån and Alsterån, that designate the wildlife and geography.