Although hoarding disorder accounts for the majority of cases of pathological hoarding behavior resulting in severe clutter, hoarding is a heterogeneous symptom that can be present in a range of organic and psychiatric disorders. DSM-5 criteria for hoarding disorder require that the hoarding behavior not be attributed to another medical condition (e.g., brain injury, Prader-Willi syndrome) or psychiatric disorder (e.g., schizophrenia, dementia). With the exception of obsessive–compulsive disorder—and perhaps obsessive–compulsive personality disorder—the clinical characteristics of hoarding in the context of other medical and psychiatric disorders have not yet been subject to much empirical research, and many questions remain open. This chapter focuses on the phenomenology of hoarding as a symptom of obsessive–compulsive disorder and hoarding secondary to other medical and mental disorders, with an emphasis on the clinical characteristics that allow for differential diagnosis with hoarding disorder.