Mindfulness apps have rapidly grown in number and are readily, indeed often freely, available to everybody with a mobile device and Internet access. They possess unique advantages for disseminating mindfulness skills compared to in-person formats. This chapter discusses the advantages and limitations of mobile and in-person formats in teaching and learning mindfulness skills, and summarizes scientific research on the effects of mindfulness apps on diverse clinical and non-clinical populations and outcome domains. It is concluded that overall, mindfulness apps have small-to-moderate positive effects on diverse mental health, well-being, and quality-of-life outcomes.