This study focused on how French children aged 4, 6, and 8 years evaluate the conditions of use for modal expressions marking certainty and uncertainty in discourse. Children were shown films involving verbal interactions during which one of the protagonists produced a target utterance accusing another character of having performed a deed. Each scenario varied in three ways: (1) presence versus absence of the speaker during the deed; (2) assertive form versus modalized form of the utterance; (3) true assertion versus false assertion. Children were asked to attribute an epistemic state of certainty or uncertainty to the speaker. Results show a clear increase from 4 to 6 to 8 years of age in the attribution of uncertainty, and a developmental progression in children's conceptions of the epistemic categories of certainty and uncertainty. Young children have a realistic conception, mainly determined by the truth/falsity of the utterance. At 8 years of age, the two other factors become more important. Children take into account linguistic markers such as modal devices, and show a metalinguistic and relativized conception of epistemic categories. (Author/DJD)