The euro is becoming a strong rival to the dollar as a top international currency. Factors like the size of a country, financial center liquidity, and return rates play a role in this shift. Changes in currency shares happen slowly, with a lag of about a year. Predictions suggest the euro could surpass the dollar as the leading reserve currency by 2015, especially if the UK doesn't join the eurozone. London is a key financial hub for the euro, even more than Frankfurt. The dollar's trend of depreciation over the last 20 years is also considered in these calculations.