There are presently more than 1,000 smartphone apps offering mindfulness or meditation in some form, marketed largely to improving one’s health and well-being. At the same time, scientific credibility of most available mindfulness and meditation apps is considered low, and clinical testing of these apps is only in beginning stages. Nevertheless, initial work is under way and suggests that evidence-based mindfulness apps may have clinical benefits for mental health. This chapter reviews the state of the science of mindfulness apps, with a focus on the treatment of substance use disorders. Mindfulness training, in person, has been shown to be effective in the treatment of substance use disorders. Mobile technologies such as smartphone apps offer numerous advantages to delivering mindfulness training, conducting clinical studies, and potentially improving clinical outcomes in substance use treatment over in-person mindfulness training. This chapter outlines the advantages and some possible limitations of delivering mindfulness training via smartphone app, reviews available mindfulness apps for mental health, reviews scientific literature supporting these apps, and proposes areas of focus for further development of mindfulness apps. Findings indicate that mindfulness training is feasible to deliver by smartphone app and other mobile technologies, initial clinical testing of mindfulness apps for substance use treatment is promising, and larger randomized controlled trials are under way. Importantly, more mindfulness apps and trials are utilizing evidence-based clinical practice guidelines.