Many of our choices are habitual. Choices such as the route you take to work every day are at first selected because they yield the best outcome (the fastest route). However, after making the same choice day after day, it becomes automatic. Habits provide a shortcut between the environment (the road) and the choice (the route to work) without using cognitive resources to weigh potential alternatives. But this shortcut means that when the choice is no longer optimal (e.g. because of roadworks) it takes effort not to perform the action anyway. This makes it difficult to make lifestyle changes (e.g. eating less sugar) and contributes to mental health problems like addiction. Animal studies show that many repetitions of the same behavior (overtraining) are needed for habit formation. However, most studies in humans focus on early parts of habit learning and few studies investigate overtraining. This project will investigate how simple choices (e.g. selecting the tastiest snack) and more complex choices (e.g. selecting the door, cupboard and ultimately the tastiest snack) become habitual after overtraining. I will use brain scanning and stimulation techniques to identify brain areas involved when acting out of habit, and which brain areas are needed to override habits. Finally, I will investigate how personality traits like impulsivity relate to how quickly a habit is formed and how difficult it is to override. This may help identify people who struggle to override habits.