In this article, we review findings on the role of negative cognitive styles and information processing biases as cognitive vulnerabilities to unipolar and bipolar mood disorders. We first briefly describe the cognitive theories of unipolar depression and the logic of their extension to bipolar disorders, as well as methodological issues involved in conducting vulnerability research, particularly with bipolar samples. We then review the evidence for negative cognitive styles, information processing, and rumination, alone and in combination with life events, as vulnerabilities to unipolar depression and bipolar disorder, as well as the developmental antecedents of cognitive vulnerability to both forms of mood disorder. In our review, we give particular attention to the findings from the Temple-Wisconsin Cognitive Vulnerability to Depression Project and the Wisconsin-Temple Longitudinal Investigation of Bipolar Spectrum Project. Our review suggests that similar cognitive vulnerabilities may increase individuals' risk for both forms of mood disorder.