Hoarding disorder (HD) is a newly listed disease in the new category of Obsessive–Compulsive and Related Disorders in the DSM‐5. Patients with HD find it difficult to discard possessions regardless of their actual value and to organize those things. As a result, the possessions overflow the living space and hinder living functions. Though the hoarding symptom had been regarded as a subtype of obsessive–compulsive disorder (OCD) to date, recent studies have revealed many differences in clinical characteristics, including onset, course, degree of insight, and treatment responses, between hoarding and other subtypes. Moreover, several neuroimaging studies have found specific changes of brain structure and function in OCD patients with hoarding symptoms compared to patients with non‐hoarding OCD. Meanwhile, strategies for treatment of HD have not been standardized. At present, psychological treatment using cognitive behavioral therapy techniques has a certain effect. In this review, we outline the pathophysiology and treatment of HD.