Mindfulness, adapted from ancient Buddhist thought and practice, was introduced into the West in a secularized and Westernized form during the 1980s. In subsequent decades, it spread around the world, into clinics, workplaces, and schools. The practice involves cultivating the ability to focus attention, and to notice any distracting thoughts and feelings without judgment or elaboration, in order to reduce stress and improve mental health. As such, it is a psychological phenomenon involving metacognition, or thinking about thinking, though this can be placed within a holistic framework that sees the mind as intricately linked with the body and the external world. In the early years of the 21st century, concerns grew about children’s mental health, and schools became seen as places to address this through universal programs; that is, mental health promotion programs that reach all students and that therefore do not stigmatize those who already have psychological difficulties, or are at risk of developing them. Evidence was also accruing that, with samples of healthy (non-clinical) adults, mindfulness had moderate effects on measures such as anxiety, and strong effects in reducing stress. Although research designs were generally not very strong, the positive results and public enthusiasm for mindfulness encouraged the introduction of universal programs into schools, and even preschools. However, the dissemination of school-based mindfulness programs ran well ahead of the scientific evidence examining their efficacy (under tightly controlled conditions) or their effectiveness in real-world school contexts. While studies were suggestive that mindfulness could affect many aspects of children’s and adolescents’ wellbeing and development, the body of research as a whole fell short in terms of scientific rigor. There were few well-designed randomized controlled trials that would enable firm conclusions to be drawn that any identified effects were due to the mindfulness program rather than to unknown factors. Moreover, little attention was paid to the presumed mechanisms of change or to the developmental appropriateness of programs. As more, and better-designed, studies began to emerge, accumulating results suggested that effects were generally small, but stronger for older than younger adolescents, and longer lasting for adolescents than for children. Issues that remained for further systematic attention included many matters of program design and implementation, the safety of the practice, its basis in developmental theory and research, and its ethical and political implications.