The comorbidity of anxiety and mood disorders has been of great interest to psychopathology researchers for the past 25 years. One topic––the comorbidity of generalized anxiety disorder (GAD) and major depressive disorder (MDD)––has received considerable attention, in part because it has raised fundamental nosological issues regarding whether GAD should continue to be categorized as an anxiety disorder or whether it should be recategorized as a mood disorder. We review the logic for reclassifying GAD with the mood disorders as well as what we believe to be even more compelling reasons for why it should be retained as an anxiety disorder. In doing so, we review three different kinds of comorbidity—cross-sectional, cumulative (lifetime), and sequential. We also discuss overlaps and distinctions in what is known about the etiology of GAD and MDD and how their somewhat different cognitive and affective profiles bear on these issues of classification. Finally, we briefly discuss what some of the treatment implications may be for individuals with comorbid GAD and MDD.