Significant cognitive dysfunction has been recognized as an important state and trait feature of bipolar disorder. In this article, longitudinal studies comparing cognitive performance in bipolar disorder patients and healthy controls are reviewed. In contrast to cross-sectional reports, current longitudinal research findings do not support a progressive cognitive decline over time. However, a higher within-person instability in cognitive performance was found relative to controls. The need for larger samples remains, as well as for longer and more frequent observations.