In the past two decades, mindfulness has gained increasing interest among researchers, clinicians, and the general public. This interest has prompted questions concerning what mindfulness is, how to measure it, and how it works. This chapter provides an overview of the nature, assessment, and mechanisms of mindfulness and explores how different approaches to these questions can inform mindfulness measurement selection. We first review common and distinct themes across Buddhist and secular mindfulness definitions, including Kabat-Zinn’s description of mindfulness as the awareness that arises through nonjudgmental and purposeful attention to the present moment. Second, we describe how the conceptualizations of mindfulness as a practice, trait, state, and process have informed diverse assessment approaches (e.g., type of assessment: trait, state, behavioral). We caution that similarly named scales may assess different constructs, while scales measuring similar constructs may be labeled differently. Finally, we describe common and distinct features of theoretical mindfulness models, which conceptualize the key mechanisms of mindfulness from various psychological, behavioral, and neurological perspectives. Mechanistic models of mindfulness describe how present-centered, nonjudgmental awareness enhances attention, emotion, and self-regulatory processes that influence numerous psychological, behavioral, cognitive, and health outcomes. We conclude by outlining additional practical considerations for measurement selection when designing/conducting mindfulness research, including research question, target outcomes, population, intervention dosing, mindfulness techniques, clinical needs, psychometric properties, and cultural relevance.