Every minute of every day, our brain’s habit system is hard at work automating well-practiced actions so our brains can focus on new and more complex challenges. This does not always work to our benefit, however. Research has implicated hyper-expression of habits in a range of compulsive behaviours, from drug addiction to out-of-control spending, binge-eating and obsessive-compulsive rituals. Despite the importance of habits in our lives, there are major gaps in our understanding of how they are acquired in humans and a virtual absence of research into how they can be broken. HABIT aims to change this by leveraging the millisecond precision of electrophysiology with the breadth of large-scale phenotyping via smartphone to develop a new mechanistic model of how habits and developed and broken, in the lab and real-life. With clear potential for impact, the fundamental insights from this project will reveal how we can harness the power of habits in our lives and better understand key aspects of mental illness.