In the heyday of the Empire—from Augustus right down to the Severan dynasty, in the mid-third century AD—the vast bulk of the Imperial coinage was struck at Rome. Hundreds of cities also struck coins, but these were almost always bronze coins of low denomination, with Greek legends, and intended for local commerce. Imperial coins were suitable for paying the army and paying taxes, and included almost all silver coins and all gold coins. Most of these were minted in Rome itself.
By the end of the third century, this system had broken down as a consequence of the general economic and political collapse. Imperial coins themselves had devolved to small, crude tokens almost devoid of precious metal content, reducing the need for lower-value local coins.
One of Diocletian's accomplishments was reorganizing the system of Imperial mints. This was based, first and foremost, on military needs. You can see a line of mints stretching from London to Serdica, to meet the needs of the many legions stationed on the frontiers, on perpetual guard against barbarians. Another group of mints served the heartland of Italy and southern Gaul; a second cluster served the traditional Greek heartland around the Aegean and western Asia Minor. The great cities of Antioch, Alexandria, and Carthage also had their own mints. Click on a city on the map to see coins from that city, or select it from the list at left.
During the reign of Constantine I, the mints got somewhat reordered. New mints were opened at Arelate (replacing Ostia and Lugdunum), and Sirmium (replacing Serdica). And of course, when he founded his new capital of Constantinople, an important mint was established there, eventually superceding that of Nicomedia. These mints are listed in blue.