Defense mechanisms are unconscious, automatic processes that allow us to cope with tension and stress. They play a significant role in maintaining mental health, but the use of some of them, especially immature ones which strongly distort reality, can be associated with psychopathological symptoms. Multiple studies show a relationship between immature defensive styles and mood disorders. Individuals with depressive and bipolar affective disorders use more immature mechanisms compared to non-clinical control groups. At the same time, they rely less on mature, adaptive defense mechanisms. Immature defense mechanisms may negatively affect the course and effectiveness of treatment, while improvements toward the use of more mature defenses due to psychotherapy and other treatment interventions are observed. Estimation of the maturity level of defense mechanisms may prove useful in the diagnostic process, especially in differentiating depressive disorders from anxiety disorders, differentiating between subtypes of mood disorders and in assessing the risk of suicidal behavior. Enhancing mature defense mechanisms and reducing reliance on immature ones may improve the overall functioning of patients with mood disorders and contribute to reducing the severity of psychopathological symptoms.