Three experiments were carried out to investigate people's concept of money. In the first experiment, subjects were asked to rate items for their typicality as money, and subsequently to say, as quickly as possible, whether or not those items were 'a kind of money'. Subjects were quicker to categorize the more typical instances. In the second experiment, subjects were asked to rate the similarities between pairs of instances of the money concept. The mean ratings were well described by a single-link cluster analysis whose structure was dominated by the factor of typicality. These two experiments were both carried out in England. The third experiment was conducted in both England and the Netherlands, and was preceded by a postal questionnaire aiming to identify items which were widely agreed to be instances of money. It combined the manipulations of the first two experiments, and the same patterns of categorization times and similarity ratings were found. In addition, multidimensional scaling showed that typical value is an important dimension in determining the similarities between kinds of money. There were interpretable differences in the similarity ratings between the English and Dutch samples. The three experiments show that money is a typical 'polymorphous concept', that is, a concept whose definition and boundaries cannot be specified precisely, but which nonetheless can be used consistently and efficiently. © 1992.