When administering an in-class exam, a common decision that confronts every instructor is whether the exam format should be closed book or open book. The present review synthesizes research examining the effect of administering closed-book or open-book assessments on long-term learning. Although the overall effect of assessment format on learning was mixed, two robust findings were that closed-book assessments led to a lower rate of forgetting (i.e. the percentage of change in information participants remembered on an initial test compared to that of the final test) relative to open-book assessments and students preferred to take open-book assessments. We also discuss possible moderators (e.g. item overlap) of the effect. Recommendations are provided to instructors who seek guidance regarding the most appropriate assessment format to adopt for their course. We conclude by outlining avenues for future research.