Monetarism is an economic theory about how the amount of money in an economy can affect the economy's growth and prices. Monetarists believe that if an economy has too much money (usually printed by the government to pay for things like wars and projects) then prices get too high, which is called inflation. On the other hand, if there is not enough money, then people won’t be able to buy things and the economy will slow down, which is called deflation. Monetarists believe that the only way to keep inflation or deflation in check is by carefully managing the amount of money in the economy.