PPP is a theory which states that exchange rates between currencies are in equilibrium when their purchasing power is the same in each of the two countries. This means that the exchange rate between two countries should equal the ratio of the two countries price level of a fixed baset of goods and services. When a countries domestic price level is increasing (i.e. a country experiences inflation) that countries exchange rate must depreciate in order to return to PPP. Therefore, PPP describes the long run behaviour of exchange rates.