The Langer Mindfulness/Mindlessness Scale (MMS) is a rationally derived 21-item self-report questionnaire, developed to measure individual differences in trait mindfulness from a socio-cognitive conceptualization of mindfulness. From this perspective, mindfulness is characterized as a process of paying attention on purpose in the present moment, which facilitates an openness to new information and possibilities, an awareness of novelty in one’s environment, and actively perceiving differences in contexts and events. The factor structure of the MMS is comprised of a single primary latent factor and four domains: Novelty Seeking, Engagement, Flexibility, and Novelty Producing. Evaluations of construct validity showed good convergent and discriminant validity with related constructs; however, the initial development of the MMS did not examine reliability. Two studies independently examined the psychometric properties of the MMS and found support for a 9-item global measure of mindfulness and a 14-item, 3-factor model. Both studies provided evidence of the validity and reliability of their respective alternative models. Currently, the MMS has been validated in English, German, Italian, Malaysian, Persian, and Chinese.