The capacity to understand one's own actions and those of others in terms of cognitive and affective mental states (i.e., reflective functioning or mentalizing) is thought to play a critical role in both typical and atypical development. To date, however, no self-report tool is available for assessing reflective functioning ability in French-speaking samples. The first aim of this study is to investigate the reliability and validity of the reflective functioning questionnaire (RFQ) in French-speaking adolescents and adults. Secondly, we investigate whether low levels of reflective functioning were associated with non-suicidal self-injury.