Teachers become rapidly more effective during the early years of their career but tend to improve increasingly slowly thereafter. This article reviews and synthesises converging evidence from neuroscience, psychology, economics and education suggesting that teachers’ rate of growth slows because their practice becomes habitual. First, we review evidence suggesting that teaching is highly conducive to habit formation and that teachers display characteristic features of habitual behaviour. Next, we review empirical findings that performance asymptotes, as seen in teachers’ learning curves, coincide with the reallocation of behaviour regulation to neural circuits governing habitual behaviour. Finally, original data is presented showing that teachers’ behaviour becomes automatic around the time that teacher effectiveness begins to level off. Collectively, this evidence implies that professional development should involve repeated practice in realistic settings in order to overwrite and upgrade existing habits.