The coinage of the Roman empire consists of many complex, and not completely understood, series. The early empire used the same denominations as the republic, and these were possibly broken down as follows:

1 aureus (gold - emperor laureate)) =
25 denarii (singular: denarius)(silver - emperor laureate)) =
100 sestertii (singular: sestertius)(bronze - emperor laureate) =
200 dupondii (singular: dupondius)(bronze - emperor radiate) =
400 asses (singular: as)(copper - emperor laureate) =
800 semisses (singular: semis)(copper) =
1600 quadrantes (singular: quadrans)(copper)

In the reign of Caracalla (198 - 217), a new coin, called the "antoninianus" was introduced. This would appear to have been a double denarius. It was made (initially) of silver, is roughly 1.5 times the size of a denarius and depicted the emperor wearing a radiate crown. The denarius depicts the emperor wearing a laurel crown. This is compared to the relationship between the as (emperor wears a laureate crown) and dupondius (emperor wears a laureate crown). The actual name of this new coin is not known, but is called the "antoninianus" after Caracalla's official name, Marcus Aurelius Antoninus.

The value of the antoninianus steadily declined from its inception. It contained less and less silver becoming a purely bronze coin during the sole reign of Gallienus (260 - 268). During the reign of Aurelian (270 - 275), the coinage was reformed. In particular, the antoninianus was upgraded to contain at least 5% silver. The resulting coin is properly termed an "aurelianianus" in honour of its creator, and carries the legend "XXI" in the exergue (the blank area beneath the picture on the reverse). This mark has caused some controversy amongst numismatists, but probably indicates a ratio of 20:1 (XX:I) copper to silver in the makeup of the coin.

A further reform was undertaken under Diocletian (284 - 305). The aurelianianus no longer displayed the "XXI" mark in the exergue.  Such coins are termed "post-reform radiates". Also introduced at this time was a slightly larger diameter bronze coin called the "follis".  Over the following two or three decades, these gradually reduced in size and are referred to as "reduced folles " or more usually, simply as "AE3s".

Diocletian also introduced a new silver coin, the "argentius", similar in weight and size to the denarius of earlier years.

Constantine the Great (307 - 337) introduced a new gold coin, the "solidus", similar in size to the old aureus, but considerably thinner. He also introduced two new silver coins, the "miliarense " valued at 1/18 of a solidus, and the "siliqua", valued at 1/24 of a solidus. He also reintroduced a follis-sized bronze coin, the "centenionalis", so called because it was valued at 100 old denarii.

From then on, the value of Roman coins becomes ever harder to guess, and coins are generally referred to by reference to their size. An AE1 is the largest of these late coins, larger than a follis, an AE2 about the size of a follis, an AE3 down to about 15mm diameter, and an AE4 anything smaller. Some very late coins (or imitations) may be only 2 or 3mm in diameter. These are generally known as minims.