The first meta-analytic review of the mindfulness-based treatment outcome literature (Baer, 2003) said nothing about the assessment of mindfulness. None of the 21 studies included in the review had attempted to measure mindfulness, presumably because no instruments for this purpose were available. Although the review concluded that mindfulness training appeared to be helpful in reducing a variety of psychological symptoms, the extent to which participants learned the mindfulness skills taught in the interventions was not investigated. The studies also shed no light on whether documented improvements in psychological functioning could be attributed to the acquisition of mindfulness skills.