Inflation results from an increase in the amount of circulating currency beyond the needs of trade; an oversupply of currency is created, and, in accordance with the law of supply and demand, the value of money decreases. Deflation is brought about by the opposite condition. In the past, inflation was often due to a large influx of bullion, such as took place in Europe after the discovery of America and at the end of the 19th cent. when new supplies of gold were found and exploited in South Africa. In modern times wars are the most common cause of inflation, as government borrowing, the increase in the money supply, and a diminished supply of consumer goods increase demand relative to supply and thereby cause rising prices.

Inflation stimulates business and helps wages to rise, but the increase in wages usually fails to match the increase in prices; hence, real wages diminish. Stockholders make gains—often illusory—from increased business profits, but bondholders lose because their fixed percentage return has less buying power. Borrowers also gain from inflation, since the future value of money is reduced. Deflation, which historically has occurred in the downward movement of the business cycle, lowers prices and increases unemployment through the depression of business. Persistent deflation in Japan, beginning in the early 1990s, resulted in a drop in consumption, record unemployment, and general economic stagnation. An unusually steep and sudden rise in prices, sometimes called hyperinflation, may result in the eventual breakdown of an entire nation's monetary system. Among the notable examples of hyperinflation have been Germany in 1923, Yugoslavia in 1993–94, and Zimbabwe in 2008.

In the United States, annual price increases of less than about 2% or 3% are not considered indicative of serious inflation. During the early 1970s, however, prices rose by considerably higher percentages, leading President Nixon to implement wage-and-price controls in 1971. Stagflation–the combination of high unemployment and economic stagnation with inflation–became common in the industrialized countries during the 1970s. The costs of the Vietnam War and the social programs of the Johnson administration, plus the oil prices increases in 1974 by the Organization of Petroleum Exporting Countries (OPEC), contributed to U.S. inflation. By the end of the 1970s the Federal Reserve raised interest rates in an attempt to reduce inflation. Following a recession in the early 1980s, there was renewed growth, somewhat lower interest rates, and a decrease in the inflation rate.

During the early 1990s, a downward business turn created an international recession—without significant deflation—that replaced inflation as a major problem; the Federal Reserve lowered interest rates to stimulate economic growth. The mid-1990s saw moderate inflation (2.5%–3.1% annually), even with an increase in interest rates. By the late 1990s, U.S. inflation was low (1.9% by 1998), despite record growth; it has tended to be somewhat higher (roughly 2%–3.5%) in subsequent years, due largely to increases in energy costs and, to a lesser degree, to large government deficits since 2001.