This study assesses children's understanding of the Italian modal verbs "dovere" (must) and "potere" (may) in their dual function of qualification of the speaker's beliefs (epistemic modality) and behaviour regulation (deontic modality). 192 children and 60 adults participated in the experiment. Children aged 3;0 to 9;2 were presented with two tasks: one assessed their understanding of modal expressions in the epistemic domain--looking for an object by following the information contained in a sentence; the other assessed their understanding of modal expressions in the deontic domain--acting according to obligations, permissions and prohibitions in the context of a game. Half of the subjects carried out the tasks in a single-sentence format and half carried out the tasks in a double-sentence format. In the single-sentence format the subjects had to follow the directions supplied by a modalized sentence; in the double-sentence format they had to follow the directions supplied by the sentence containing the stronger modal verb. Our results show that the understanding of deontic modal forms precedes the understanding of epistemic modal forms and that a full understanding of the strength of different modal forms is achieved only at eight years.