The first part of the paper discusses empirical results published in the recent literature related to the weak form of the efficient market hypothesis. The second part of this paper deals with patterns in individual investor behavior. Based on a short literature survey the author describes basic behavior patterns in contrast to the efficient market hypothesis: investors trading on the basis of price changes, because they believe that they can improve their chances of making a profit by doing so. An experimental design is used to analyse the individual investor behavior in a more detailed way than would be possible in real financial markets. We find strong evidence for behavior patterns: subjects using historical returns to derive their investment decisions. One part of the investors might think that winning stocks will fall and losing stocks will rise again (the behavior caused thereby called tracking); another group of subjects might think that winning stocks will continue to rise and losing stocks will fall further on (the behavior caused thereby called chasing). The results can either be explained by subjects' misperception of probabilities of future price changes or by an asymmetric valuation of gains and losses (,,loss aversion").