This chapter discusses the reliability, validity, and utility of language-based measures of mindfulness (LBMM). The instrument was designed to assess an individual’s mindfulness qualities such as acceptance and the ability to act with awareness in their lives. In this measure, mindfulness is assessed by observing how frequently people use mindfulness-related language when writing responses to open-ended questionnaire items. The ability to use words as a descriptive mechanism may also be indicative of a “de-centered” or “meta-cognitive” perspective, which correlates with mindfulness outcomes and relates to the training of the brain. The efficacy of the LBMM was examined and validated using participants from an 8-week Mindfulness-Based Relapse Prevention program. Research findings showed that the use of mindfulness-related language significantly increased participant’s acceptance and ability to act with awareness and reliably predicted alcohol and drug use days at follow-up.