<p>Behaviours we repeat often tend to become habitual. The process of habit formation in humans is commonly studied in the context of learning an arbitrary association between stimuli and responses. Assessing whether or not this association becomes habitual usually involves testing either whether participants can withhold certain responses or whether participants can generate different responses to certain stimuli. However, studies using these two methods have yielded conflicting findings. Here we propose that this discrepancy arises because these approaches target distinct forms of habit associated with different components of action control: response initiation and response preparation, respectively. Through a series of experiments (total <i>n</i> = 215), along with computational models, we show that these two approaches indeed measure dissociable forms of habit. Our results illustrate that a given behaviour can become habitual in multiple, qualitatively different ways, with important implications for how habits can best be promoted or eliminated.</p>