PURPOSE: Mindfulness has emerged as an important health concept based on evidence that mindfulness interventions reduce symptoms and improve health-related quality of life. The objectives of this study were to systematically assess and compare the properties of instruments to measure self-reported mindfulness. METHODS: Ovid Medline(®), CINAHL(®), and PsycINFO(®) were searched through May 2012, and articles were selected if their primary purpose was development or evaluation of the measurement properties (validity, reliability, responsiveness) of a self-report mindfulness scale. Two reviewers independently evaluated the methodological quality of the selected studies using the COnsensus-based Standards for the selection of health status Measurement INstruments checklist. Discrepancies were discussed with a third reviewer and scored by consensus. Finally, a level of evidence approach was used to synthesize the results and study quality. RESULTS: Our search strategy identified a total of 2,588 articles. Forty-six articles, reporting 79 unique studies, met inclusion criteria. Ten instruments quantifying mindfulness as a unidimensional scale (n = 5) or as a set of 2-5 subscales (n = 5) were reviewed. The Mindful Attention Awareness Scale was evaluated by the most studies (n = 27) and had positive overall quality ratings for most of the psychometric properties reviewed. The Five Facet Mindfulness Questionnaire received the highest possible rating ("consistent findings in multiple studies of good methodological quality") for two properties, internal consistency and construct validation by hypothesis testing. However, none of the instruments had sufficient evidence of content validity. Comprehensiveness of construct coverage had not been assessed; qualitative methods to confirm understanding and relevance were absent. In addition, estimates of test-retest reliability, responsiveness, or measurement error to guide users in protocol development or interpretation of scores were lacking. CONCLUSIONS: Current mindfulness scales have important conceptual differences, and none can be strongly recommended based solely on superior psychometric properties. Important limitations in the field are the absence of qualitative evaluations and accepted external referents to support construct validity. Investigators need to proceed cautiously before optimizing any mindfulness intervention based on the existing scales.