Hoarding disorder (HD) is a new disorder included under an equally new Obsessive–Compulsive and Related Disorders section in the fifth edition of the American Psychiatric Association's (APA) Diagnostic and Statistical Manual of Mental Disorders. The authors summarize the process that led to the birth of the new disorder and describe its diagnostic criteria, its accompanying specifiers, and its ancillary dimensional rating scale. The differences between hoarding and normative collecting are briefly outlined. The authors then describe recent efforts to empirically validate the diagnostic criteria, specifiers, and rating scale. The process of diagnosing cases with HD is then described, with emphasis on the use of a semistructured interview approach, ideally carried out in the person’s own home. Future directions are discussed, including the need for further refinement of the diagnostic boundaries of HD and the validation of the criteria in other cultures and age groups.