The aim of this study is to investigate children's understanding of some forms of the Italian modal verbs "dovere" (must) and "potere" (may, can) in their dual function of behaviour regulation (deontic modality) and qualification of speaker's beliefs (epistemic modality). Participated in the experiment 192 children and 60 adults. Children were divided into four age groups of 48 subjects each: 3;0-4;6, 4;7-5;10, 6;3-7;2 and 8;3-9;2 years-olds. Subjects were presented with two tasks. In the first task subjects had to find an object following the information of a sentence containing a modal verb in the epistemic domain. In the second task subject had to act according to obligations, permissions and prohibitions expressed by a sentence containing a modal verb in the deontic domain. Half of the subjects carried out the tasks in a single-sentence format and half of the subjects carried out the tasks in a double-sentence format. In the single-sentence format children had to act according to the modal verb of the sentence. In the double-sentence format subjects had to act according to the sentence with the strongest modal verb. Results show that the understanding of the deontic modal forms precedes the understanding of the epistemic modal forms and that a full comprehension of the strength of different modal forms is achieved only at the age of 9.