Controlled deep breathing, often termed diaphragmatic or slow breathing, is increasingly recognized as a scientifically supported, non-pharmacological strategy for enhancing both physical and mental health. Growing empirical evidence demonstrates its capacity to regulate the autonomic nervous system, reduce stress-related physiological activation, stabilize cardiovascular function, optimize respiratory efficiency, and support emotional regulation. Through stimulation of vagal pathways and attenuation of sympathetic overactivity, slow breathing promotes systemic equilibrium. This review critically examines current literature regarding the biological mechanisms, psychological outcomes, clinical applications, and research limitations associated with deep breathing practices. The evidence strongly supports incorporating structured breathing interventions into preventive medicine, mental health treatment, and rehabilitative care.