Until the beginning of the 20th century, the idea of obsession found in the literature was mostly a broad and unspecific one, and it has, to some extent, persisted in this form until the present day. At the same time, however, Esquirol and Westphal began to develop a concept of obsession in the strict sense, and Jaspers used the phenomenological method as a basis to give it its final form. The concept of obsession in the strict sense represents the formal and nosologically specific core symptom of obsessive-compulsive disorder. In conjunction with the distinction between primary and secondary obsessions, obsession in the strict sense is proving helpful in differential diagnosis, but it can also serve as a guideline in the further phenomenological investigation of obsessive-compulsive disorder. It is shown how the interaction between personality, affect and release situation develops the obsessive-compulsive disorder. The clarification of these aspects with the patient can at the same time be the starting point of psychotherapeutic treatment.