Abstract I briefly review empirical data about the generalization of acquired behaviour to novel stimuli, showing that variations in stimulus intensity affect behaviour differently from variations in characteristics such as, for instance, visual shape or sound frequency. I argue that such differences can be seen already in how the sense organs react to changes in intensity compared to changes in other stimulus characteristics. I then evaluate a number of models of generalization with respect to their ability to reproduce intensity generalization. I reach three main conclusions. First, realistic stimulus representations, based on knowledge of the sense organs, are necessary to account for intensity effects. Models employing stimulus representations too remote from the sense organs are unable to reproduce the data. Second, the intuitive notion that generalization is based on similarities between stimuli, possibly modelled as distances in an appropriate representation space, is difficult to reconcile with data about intensity generalization. Third, several simple models, in conjunction with realistic stimulus representations, can account for a wide array of generalization phenomena along both intensity and non-intensity stimulus dimensions. The paper also introduces concepts which may be generally useful to evaluate and compare different models of behaviour.