Third-wave psychotherapies take a functional and contextualistic philosophical stance and emphasize the process through which adaptive and maladaptive human functioning is originated, developed, maintained, and changed. Mindfulness is one of the key elements underlying third-wave psychotherapies, with a focus on the development of effective and flexible repertoires of mindsets and behaviors. Although research has demonstrated the myriad benefits of mindfulness, the recent interest has often led to its oversimplification. The danger is that mindfulness can quickly turn into a caricature of itself, losing the nuance and complexity that make it so powerful. This chapter, in contrast, offers a comprehensive and clear understanding of mindfulness, with the aim of operationalizing it so as to deepen the discussion of its applications to third-wave psychotherapies.