Many patients with hoarding disorder (HD) also meet criteria for additional diagnoses. Although hoarding symptoms have historically been linked to obsessive–compulsive disorder (OCD), comorbid OCD afflicts only a minority of individuals with HD. In contrast, depressive disorders are the most common co-occurring conditions in HD. Other commonly comorbid conditions include anxiety disorders (especially social phobia and generalized anxiety disorder), impulse control disorders, and attention-deficit/hyperactivity disorder (ADHD). This chapter reviews the patterns of comorbidity commonly seen in patients with HD. After discussing the rates and clinical impact of comorbidity on hoarding symptoms, the authors consider additional features of comorbidity, including evidence for gender differences in comorbidity patterns and the impact of comorbid conditions on hoarding treatment.