This article reviews the literature on compulsive hoarding, including the definition and manifestations of the problem and a conceptual model for understanding hoarding behavior. This model addresses information processing deficits (e.g., attention, organization, memory, decision-making), beliefs about and emotional attachments to possessions, and distress and avoidance. Research regarding the diagnostic categorization of hoarding, its course and phenomenology, and evidence to support the model is presented. The limited research on treatment provides evidence that current serotonergic medications for OCD are largely ineffective for treating hoarding, but cognitive and behavioral treatments, especially those focused on deficits identified in the model, have some utility. Recommendations for further research on the psychopathology and treatment of hoarding are provided.