The history of research on hoarding behavior has been very brief, spanning only about 20 years. Accordingly, this volume provides the first comprehensive compendium of the most current research and clinical understanding of hoarding and the new formal DSM-5 (American Psychiatric Association, 2013) diagnosis of hoarding disorder (HD). This disorder is remarkable among mental health conditions for its adverse impact on sufferers, on family members, friends and neighbors, and on human service providers trying to address the housing, public health, and other needs of individuals with HD. This chapter provides commentary on recent discoveries and likely next steps in the multiple efforts to better understand, respond, and effectively intervene to resolve this complex condition. In this rapidly moving research field, our goal will be to update these chapters online in the coming years as new research comes on the scene.