||Irrationality is a phenomenon that can be seen in every aspect of human life. In many cases people do not behave rationally in terms of economic theory and they often do not even realise it. This thesis aims at problem of irrationality in the perception of money and describes anomalies in rational decision-making. First, it introduces the topics that behavioural economics is concerned with and its place among other sciences. It brings the overview of history of behavioural economics and the most influential people that are responsible for its evolution. Next, it briefly summarizes the most common research methods used. After that, the concept of rationality is described, both from classical economics and behavioural points of view. The core of the thesis is the experiment that is an example of irrationality in the perception of money. Studies that examined this problem are presented. Finally, a conducted survey is presented, which detects the presence of irrational behaviour.