This document provides an overview of the quantity theory of money. It discusses the history of the theory as outlined by Irving Fisher in 1911. Fisher examined the link between the money supply (M), price level (P), and aggregate output or income (Y). This relationship is captured in the equation of exchange: M x V = P x Y, where V is the velocity of money, or how quickly money circulates in the economy. The document then explains that according to the quantity theory, changes in the money supply will only affect the price level as long as velocity and output remain constant in the short run. Finally, it provides graphs showing how velocity has changed over time for different monetary aggregates in Egypt.