Reflective functioning or mentalizing is the capacity to interpret both the self and others in terms of internal mental states such as feelings, wishes, goals, desires, and attitudes. This paper is part of a series of papers outlining the development and psychometric features of a new self-report measure, the Reflective Functioning Questionnaire (RFQ), designed to provide an easy to administer self-report measure of mentalizing. We describe the development and initial validation of the RFQ in three studies. Study 1 focuses on the development of the RFQ, its factor structure and construct validity in a sample of patients with Borderline Personality Disorder (BPD) and Eating Disorder (ED) (n = 108) and normal controls (n = 295). Study 2 aims to replicate these findings in a fresh sample of 129 patients with personality disorder and 281 normal controls. Study 3 addresses the relationship between the RFQ, parental reflective functioning and infant attachment status as assessed with the Strange Situation Procedure (SSP) in a sample of 136 community mothers and their infants. In both Study 1 and 2, confirmatory factor analyses yielded two factors assessing Certainty (RFQ_C) and Uncertainty (RFQ_U) about the mental states of self and others. These two factors were relatively distinct, invariant across clinical and non-clinical samples, had satisfactory internal consistency and test–retest stability, and were largely unrelated to demographic features. The scales discriminated between patients and controls, and were significantly and in theoretically predicted ways correlated with measures of empathy, mindfulness and perspective-taking, and with both self-reported and clinician-reported measures of borderline personality features and other indices of maladaptive personality functioning. Furthermore, the RFQ scales were associated with levels of parental reflective functioning, which in turn predicted infant attachment status in the SSP. Overall, this study lends preliminary support for the RFQ as a screening measure of reflective functioning. Further research is needed, however, to investigate in more detail the psychometric qualities of the RFQ.