The object of this study is to describe the behaviour of heroin users from an economic point of view in such a way as to enable tracing the influence of policy measures. In order to describe the heroin users' behaviour, data have been collected regarding relevant variables and relations between variables by interviewing 80 heroin users in the Netherlands. Based on these data a computer model has been developed. The objective of the model is two-fold: 1) Describing the behaviour of a group of heroin users in connection with the process of obtaining money and heroin in order to determine the factors which influence the quantity of heroin bought and consumed. 2) Carrying out some scenario analyses in order to examine the effects policy changes might have on variables such as heroin use and quantity of money obtained illegally.