Mood disorders encompass a range of psychiatric conditions such as major depressive disorder, bipolar I and II disorders, persistent depressive disorder (dysthymia), and cyclothymia, characterized by sustained disruptions in emotional regulation. This chapter delivers a comprehensive, nursing-focused examination of these disorders, covering diagnostic frameworks and their implications for tailored interventions. Taking a biopsychosocial perspective, it explores the multifactorial origins of mood disorders, including genetic predispositions, neurochemical imbalances, psychological vulnerabilities, and sociocultural stressors. Clinical manifestations across depressive, manic/hypomanic, and mixed states are described to support accurate assessment. Emphasis is placed on nursing assessment tools and strategies such as mental status examination and the use of validated scales, alongside holistic evaluations informed by the nursing process. Nursing interventions are outlined at each stage: acute stabilization, safety planning, psychoeducation, therapeutic communication, fostering routine and structure, collaboration with interdisciplinary teams, and relapse prevention. This chapter aims to equip nurses with evidence-based, person-centered approaches for managing mood disorders across diverse care settings.