Since its introduction to the behavioral science research community 25 years ago, interest in mindfulness has burgeoned. Much of that interest has been among clinical researchers testing the efficacy of mindfulness-based or mindfulness-integrated interventions for a variety of conditions and populations, and this volume is testament to the vitality of investigation and diversity of applied knowledge that now exist in the field. In the last 5 years or so, researchers have also become interested in describing and operationalizing the mindfulness construct itself. This more recent line of work is important for four reasons: The first concerns the basic scientific principle that a phenomenon can be studied only if it can be properly defined and measured.