This chapter describes the rationale, development, and validation of the Observed Mindfulness Measure (OMM), a psychometrically valid research questionnaire for collecting second-person ratings of a known individual’s mindful behaviors. The nine-item OMM provides an overall score and separate scores for each of its three subscales (awareness, acceptance, and attentiveness). The OMM is intended to provide observer data with which to triangulate, and thereby augment findings in mindfulness research that are based on data from self-report questionnaires. The measure can also be used to advance the investigation of social and performance outcomes associated with trait mindfulness and mindfulness training. The chapter concludes with a discussion of planned refinements and potential future applications of the OMM.