The concept of defense refers to automated psychological processes that people use to exclude unpleasant feelings or internal and external conflicts from their consciousness. These processes can serve a protective function and be adaptive. However, when they are maladaptive, rigid, or interpersonally oriented, they can also contribute to the development and course of mental disorders. The phenomenon of defense is therefore central to the clinical understanding of mental illnesses as well as to the psychotherapeutic process. Leonie Kampe examines the psychodynamic concept of defense by first exploring different definitions of the concept from a historical and developmental theoretical perspective, and finally describing the various levels of adaptation of defense. She then highlights the clinical and practical significance of the concept of defense by identifying specific defense processes related to different disorders through illustrative case examples, presenting various functional levels of defense, and outlining diagnostic challenges and methods. The detailed presentation of difficult therapeutic situations in relation to defense facilitates the handling of these issues.