Abstract People often have good intentions but fail to adhere to them. Implementation intentions, a form of strategic planning, can help people to close this intention–behavior gap. Their effectiveness has been proposed to depend on the mental formation of a stimulus–response association between a trigger and target behavior, thereby creating an “instant habit.” If implementation intentions do indeed lead to reliance on habitual control, then this may come at the cost of reduced behavioral flexibility. Furthermore, we would expect a shift from recruitment of corticostriatal brain regions implicated in goal-directed control toward habit regions. To test these ideas, we performed a fMRI study in which participants received instrumental training supported by either implementation or goal intentions, followed by an outcome revaluation to test reliance on habitual versus goal-directed control. We found that implementation intentions led to increased efficiency early in training, as reflected by higher accuracy, faster RTs, and decreased anterior caudate engagement. However, implementation intentions did not reduce behavioral flexibility when goals changed during the test phase, nor did it affect the underlying corticostriatal pathways. In addition, this study showed that “slips of action” toward devalued outcomes are associated with reduced activity in brain regions implicated in goal-directed control (ventromedial prefrontal cortex and lateral orbitofrontal cortex) and increased activity of the fronto-parietal salience network (including the insula, dorsal anterior cingulate cortex, and SMA). In conclusion, our behavioral and neuroimaging findings suggest that strategic if–then planning does not lead to a shift from goal-directed toward habitual control.