Is hoarding a mental disorder? How common is it?

Is hoarding considered a mental disorder? How common is hoarding and how do you know if someone is a hoarder versus simply being cluttered and unorganized?

Hoarding has been considered a subset of Obsessive/Compulsive disorder.  The new edition of the Diagnostic and Statistical Manual, the DSM-5, will treat it as an illness in its own right. All mental disorders are only clinically diagnosable when they impair, or significantly impact, academic, occupational and/or social functioning. Hoarding would have to be impeding one’s ability to work, have satisfying relationships, or become a health risk in order to be diagnosed as an illness.