Training attention and awareness in mindfulness meditation is theorized to be essential for the cultivation of mindfulness and its salutary outcomes. Yet, the empirical foundation for this central premise in mindfulness science is surprisingly small due to a limited methodological capacity to measure attention and awareness during mindfulness meditation. Accordingly, we set out to measure and study these processes in a laboratory study (N = 143, 76.92% female) using a novel behavioral measurement paradigm-the mindful awareness task (MAT). We empirically characterized attention and awareness during mindfulness meditation and found novel behavioral evidence indicating that, as long-theorized, these processes were related to previous mindfulness meditation practice, attitudinal qualities of mindfulness, attention regulation, and mental health. Furthermore, we found that the accuracy of self-reported mindfulness was, paradoxically, dependent on behavioral capacities for mindful awareness; and that sustained visual attention and executive functions, measured via cognitive-experimental tasks, were not meaningfully related to attention and awareness during mindfulness meditation. In contrast, the MAT demonstrated sound psychometric performance as a measure of mindful awareness, and may overcome significant limitations of extant mindfulness measurement methods. Collectively, findings challenge conceptual and methodological assumptions in mindfulness science, provide a novel paradigmatic direction for research on mindfulness, and present long-awaited evidence that attention and awareness during mindfulness meditation may indeed be fundamental to its practice, cultivation, and salutary functions. Public Significance Statement Mindfulness meditation is implemented in a variety of settings and applications. Yet we lack empirical evidence, and methods to study, a central premise of this intervention approach-that training attention and awareness through mindfulness meditation is essential to its practice and salutary functions. Using a novel behavioral measurement paradigm, we provide long-awaited support for this central premise and show that this paradigm may help advance empirical understanding of mindfulness.