Objective: To compare the predictive power of goal intentions and implementation intentions. Design: Both constructs were expected to explain physical exercise at 3 points in time (with follow-ups at 6 and 12 months) in a sample of 368 persons participating in orthopedic rehabilitation. Results: Goal intentions and implementation intentions predicted exercise during rehabilitation. In contrast, goal intentions failed to predict exercise at later points in time, whereas implementation intentions continued to be associated with exercise 12 months later. Conclusions: Implementation intentions rather than goal intentions predict behavior as it becomes routine. As automatic processes such as behavior elicited by planning are largely age invariant, strengthening those processes might help to overcome volitional problems across all age groups represented in rehabilitation settings.