This article offers a practice-friendly review of conceptualizing and treating hoarding based on biological and psychological research. Although hoarding has commonly been conceptualized as a variant of obsessive-compulsive disorder, research suggests a more complex pattern of overlap with attention deficit-hyperactivity disorder, organic brain disease, depression, anxiety, personality disorders, and impulsivity. The phenomenon of hoarding is discussed in terms of familial and environmental vulnerabilities, maladaptive cognitive process, maladaptive cognitive content, and maladaptive behavioral patterns. The article concludes with a summary of treatment efficacy and 10 promising clinical practices. © 2011 Wiley Periodicals, Inc. J Clin Psychol: In Session 67:1–10, 2011.