From a psychological point of view, this study looks at children’s and adult’s comprehension of idiomatic expressions, and most particularly at the underlying cognitive processes needed for comprehension. Idiomatic expressions are expressions where there is a considerable difference between what is said (literal interpretation) and what is meant (idiomatic interpretation). In other words, the meaning of an idiomatic expression depends largely on a convention that relates a given linguistic form to a given meaning. Conducted in this framework, the present study was aimed at determining the role of contextual characteristics and the linguistic convention in the comprehension of idiomatic expressions by 6- and 9-year-old children, and by adults. The subject’s task was to complete stories. Twelve stories were presented in comic strip format by the experimenter, who told the story in the first three frames and then instructed the subject to choose one of the two possible endings. Two features of the stories were varied : The utterance production context (idiomatic vs. literal) and the idiom familiarity level (familiar vs. unfamiliar). Regardless of age, the context had a substantial impact on idiom comprehension: This reinforces the idea of the necessity of taking the context and the extra-linguistic conventions into account in order to explain language functioning, not only in children but also in adults. The role of the linguistic convention began at the age of 9 and was particularly strong in adults: They appear to reconstruct the communication situation solely on the basis of the linguistic convention. The role of familiarity also appeared in the 9-year-olds and continued on into adulthood. These results suggest that the period of adolescence is crucial for the development of certain pragmatic aspects of language.