The development of modal meanings has been an important topic in the literature on semantic change and evidence exists for a diachronic relation between root and non-root (epistemic) meanings. However, there is less certainty regarding the details and potential causes of this development. This chapter contributes to this discussion by considering the development of the Swedish modal må 'may', arguing that the concept of force-dynamics can explain the relations among different modal meanings. The assumption is that a general image-schema in terms of forces and barriers underlies and motivates the root as well as the non-root meanings. Morever, I show how the schema may be transferred to and used in different kinds of semantic/conceptual domains.