Imagining one's possible future self can motivate action but whether motivational power resides more in positive or more in negative future identities is not clear. We predicted that motivational power resides not in these positive or negative future identities but in the fit between context and future self. We varied fit in four experiments by having students read about college as a success-likely or failure-likely context and then write about their desired or undesired possible future identities. Which aspect of the future self was motivating depended on context. Motivation was higher in success-likely contexts if desired rather than undesired possible futures came to mind and was higher in failure-likely contexts if undesired rather than desired possible futures come to mind.