Introduced as a small silver coin in 211 BC, along with the denarius, its initial value was two and a half asses, or 1/4 of a denarius, but was changed to a very large bronze coin after 44 BC which was worth four asses. The sestertius was produced as the largest brass denomination but the coin shrunk in size slowly and disappeared in the 3rd century at which time it was made of bronze and was rare.
This is a large coin, more than the US half dollar (30mm+) in size, and often shows great detail. During the Empire it was struck from a bronze alloy called orichalcum, making it look yellowish like the smaller dupondius, however, the emperor is always shown wearing a laurel wreath which helps to distinguish it. It is also distinguishable from the as due to its larger size.