It's called "Arbitrage" and it is why the various currency exchanges tend to have similar prices.  Whenever prices get too different between any two or more exchanges, an arbitrage opportunity exists.  Some people take advantage of the arbitrage opportunity. By buying the bitcoins at the exchange where the price is too low, they drive the price up. By selling the bitcoins at the exchange where the price is too high, they drive the price down.  This occurs until the price difference is no longer large enough to cover the cost of the fees and the risk of the price moving while you wait for your funds to transfer between exchanges.