Over the past two decades, investigations of mindfulness meditation have demonstrated considerable efficacy in reducing the symptom burden associated with a variety of medical and mental health disorders (Baer, 2003). This chapter reviews the theoretical basis for offering training in mindfulness meditation to these populations, and it outlines the structure of mindfulness-based interventions, as well as their impact on stress and psychological indices of mental and physical health. We first define mindfulness in terms of the core cognitive processes that are engaged through this practice and then review how mindfulness reduces ruminative and elaborative processing, factors known to perpetuate stress reactivity. From there, we describe the dominant theoretical model of mindfulness’s impact on stress-related disorders—the mindfulness stress-buffering account (MSBA; Creswell & Lindsay, 2014)—and highlight how using this framework can inform intervention science in the area of stress reactivity.