The paper outlines the phenomenon of currency crises. It also describes a model of a currency crisis resulting from herding behaviour. The model is based on the assumption that there are two investors on the market, who maximise withdrawals and decide whether to purchase a currency or to maintain the status quo. The decisions are made on considering market feedback and the actions taken by the other investor. This model is then expanded to cover a larger number of investors. The conclusions of the analysis constitute the thesis of this article. The more imperfect is the market information and the more numerous are investors, the stronger the herding behaviour. A currency crisis may also occur even in the absence of sufficient impulses.