Exchange rates
The movements of exchange rates can have a dramatic affect on the gains or losses of your stock investments if you pick stocks in a different country. For example if I am a US investor and I invest in a UK company I am exposed to the exchange rates between the US dollar and the UK pound. If the value of my shares rises by 5% but the exchange rate moves against me in the same period of time by 6% then if I sell I will have made a net loss, despite the value of my shares increasing.