The article discusses interpretation of between-group differences in performance on timing tasks. First, it is shown that differences in internal clock "pacemaker speed" cannot normally be used as a coherent explanation of obtained effects, even if such differences in pacemaker speed exist. Secondly, it is shown how, in theory, modelling of performance on commonly used timing tasks like bisection and temporal generalization can illuminate between-group effects. Thirdly, the article discusses some examples of such modelling from published work and shows how some between-group differences--for example, between children of different ages, or between patients and controls--have been explained. Finally, some ambiguities in modelling are discussed--for example, the fact that different explanations of differences in performance on timing tasks between groups may be difficult or impossible to distinguish in practice.