The traditional approach to the theory of finance stipulates that the capital market should be effective, i. e. that the data that influence the value of securities should be reflected by their prices while the rate of return should solely be dependent on risk levels. Behavioural approach to finances, dwelling on attainments in experimental psychology, puts into question numerable premises that have, until recent times, been widely accepted by the theory of finance. This new standpoint changes the perception of financial market processes. It has been found that behavioural finances better explain the phenomena that fail to be explained by the traditional capital market theory. It remains to be seen whether behavioural finance evolves into a new theory, allowing not only for the explanation of past events, but also enabling to build better models that would predict actions of investors.