Hoarding disorder is characterized by difficulty discarding or parting with possessions regardless their actual value as well as, in most cases, persistent acquisition of objects. Possessions are accumulated in large numbers that fill up and clutter active living space to the extent that its intended use is no longer possible resulting to significant functional impairment and/or distress. Saving and difficulty discarding appear to be associated to subjective beliefs about the instrumental, sentimental or intrinsic value of objects. Despite significant impact on the individual, the family and the community, until recently hoarding disorder had been a "neglected" clinical phenomenon. The recent research efforts to study and understand this phenomenon led to significant changes regarding the diagnostic status and therapeutic interventions. It was classified as a distinct diagnostic entity in the recent edition of the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association-5th ed. (DSM-5). Hoarding is not considered a symptom of obsessive compulsive disorder or obsessive compulsive personality disorder anymore and it is now ranked among compulsive spectrum disorders. So far the cognitive model for hoarding disorder is the main, widely accepted, clinically and empirically supported theoretical framework for understanding its features. The cognitive model offers an explanation about the predisposing, onset and perpetuating factors of the disorder as well as the basis for the development of specific therapeutic interventions. The cognitive model conceptualizes hoarding disorder as a result of four factors: personal vulnerability (heredity, early experiences and life events, personality traits, interpersonal difficulties), difficulties in information processing (deficits in attention, memory, executive functions such as decision making and categorization), maladaptive cognitive content (meaning of possessions, emotional attachment to possessions, dysfunctional beliefs in relation to the mnemonic ability and memory importance) through which hoarding behaviors are reinforced and finally installed (positive and negative reinforcement, association with positive or negative emotions e.g. pleasure in relation to acquisition/saving and anxiety or discomfort in relation to discarding). The purpose of this article is to inform about this new disorder giving emphasis on recent data in regard to its phenomenology and provide a detailed description of the cognitive model. Finally, a brief mention on the psychotherapeutic and pharmacological interventions is presented. Hoarding Disorder is a difficult clinical problem to treat. Poor insight, ego-syntonic nature of symptoms and treatment resistance are often reported in patients. Further improvement of the therapeutic interventions in order to improve effectiveness and target the high rate of treatment drop out is needed.