Diocletian's Maximum Prices Edict was the second of two laws instituted in 301CE as part of an Imperial attempt to regulate the economy and curb the inflation that was rampaging across the Roman Empire (which was partly a result of the Emperor Caracalla debasing the coinage in 215CE and continual debasements from 238-270CE, in addition to increased taxation to support the expanding army).

Whether or not the Romans actually had a concept of inflation is debated; Primitivists (for example M. Finley and A.H.M. Jones) suggest that the Roman economy was underdeveloped and badly understood, whilst Modernists (such as M. Rostovtzeff) argue in favour of a more sophisticated system that was better comprehended. Regardless, in 301CE, prices were rising at a remarkable rate, forcing the Emperor to take action. The first economic edict of 301CE concerned the regulation of the currency, which involved some coins being doubled in value and the new tariff being used for the repayment of all debts. However, there was little public confidence in the new monetary system, hence the introduction of the Maximum Prices Edict.

The edict laid down the maximum prices that could be charged for some 1,000 goods and services, across the entire Empire. The range of goods was extensive, including cereal grains, rice and pulses, meats and fish, oil and wine. Different quality products were also priced accordingly, for example, Gallic or Pannonian beer was more expensive than Egyptian (4 denarii compared to 2 denarii for 1 Italian sextarius, which is equivalent to a little under 1 pint, or 546ml). Teachers of rhetoric or public speaking could expect to earn more than elementary teachers, fees being fixed at 250 denarii per month per pupil for the former, 50 denarii per month per pupil for the latter.

The punishment for failure to abide by the edict was capital, the preamble of the edict stating that: "And let no one consider the statute harsh, since there is at hand a ready protection from danger in the observance of moderation..." However, the edict did not really produce the desired effect. Not only was it badly received by the public, but it also had counter-productive economic effects. By implementing what was essentially a universal pricing policy, regional price variations disappeared; consequently, trade suffered. Furthermore, it made some products unprofitable to sell, so people stopped selling them.

Despite these oversights, it does appear that Diocletian did have a grand plan when he implemented the edict. He was trying to produce a budget for the Empire, based on a five-yearly tax assessment. At this stage, the Empire's borders were under significant strain and required serious military support. In order to finance an ever-increasing army, taxes were required. Diocletian had a balance sheet: the sum necessary to sustain the army against the sum that could feasibly be drawn from the Empire in taxation. However, this plan was only workable if prices remained consistent and similar taxes could be raised every year. It was not only an issue of stabilising the economy, but also the Empire itself.

One of the sources for the edict (for there are several, having been an Empire-wide decree, copies were dispatched extensively) is a marble tablet found in the remarkably well-preserved city of Aphrodisias, modern Geyre, Turkey. The condition in which the tablet was found is perhaps indicative of the loathing that Diocletian's subjects had for the edict: it had been smashed, with a hammer, or similar!