The Roman Empire came after the Roman Republic. It is generally considered to have began when Octavian assumed the title of "Augustus". During this time typically the bust of the emperor was on the obverse. This period usually is marked at an end in 395 AD when Constantine the Great moved the Roman capital to Constantinople. The coinage during these several hundre years changed a lot with the changing of the rulers and many other factors. Of all of the various changes and types of coins made the late Roman coins are often first for collectors because of their availablity and price point.