The Japanese yen has strengthened over the past 20 years due to it being a currency with net inflows. More yen are bought than sold, leading to increased demand. This strengthening tendency is reinforced further by low returns on investments in other countries' foreign reserves, Japan's trade surplus, and monetary policies in the US and Europe. Historically, the yen has fluctuated but the overall pattern has been for it to grow stronger against the dollar. The strength of a currency is driven by balances of supply and demand - when demand exceeds supply, as it does currently for the yen, the currency strengthens in value.