The Mindfulness Inventory: Nine Dimensions (MI:ND) provides a precise and comprehensive assessment tool relevant to understanding how mindfulness may affect psychological and behavioral outcomes. The MI:ND includes multidimensional measures of: (1) the core components of mindful attention and (2) proximal outcomes of mindfulness related to changes in emotion regulation. Core mindfulness dimensions include: Present awareness, Willingness/acceptance, Metacognition, Quiet mind, Non-judgment of self, and Non-judgment of others. Proximal outcomes of mindfulness include: (low levels of) Experiential avoidance, Distress tolerance, and (low levels of) Emotion-driven impulsivity. The MI:ND was initially validated in a sample of university students. Subsequently, the MI:ND has been used in empirical research in further samples of university students and also jail inmates. Evidence of psychometric properties from these samples is presented, including validity and reliability. Eight of the nine subscales have demonstrated adequate internal consistency across samples. The Metacognition subscale has shown poor internal consistency but adequate test-retest reliability and concurrent validity. Evidence to date suggests that the MI:ND may be a useful comprehensive measure of mindfulness assessing both core features of mindfulness and proximal outcomes in the domain of emotion regulation – theoretical mechanisms of action potentially explaining the benefits of mindfulness.