Measuring adherence in mindfulness-based interventions is commonly subject to two significant omissions. First is not measuring the quality of practice, and second is not measuring the relative contributions of formal versus informal mindfulness practice. The 12-item self-administered Mindfulness Adherence Questionnaire (MAQ) was designed to measure the duration and frequency of formal meditation practice and also the quality of formal and informal practice. The MAQ was validated with medical students and demonstrated good validity and reliability. The MAQ will be of use to mindfulness researchers and practitioners alike and will help to measure the level of participant engagement and to distinguish which aspects of mindfulness practice make the most significant contribution to overall outcomes.