Diabetes mellitus comprises a heterogeneous group of metabolic disorders differing in etiology, clinical course, and outcomes. Traditional classifications, such as type 1 and type 2 diabetes mellitus, fail to capture the full heterogeneity, including variation in insulin deficiency, insulin resistance, and complication burden. To address these limitations, we propose the Diabetes Grade-Stage Classification, an integrated system that combines pathophysiology-based grading with complication-based staging. Grading quantifies metabolic dysfunction through the assessment of insulin deficiency and insulin resistance. In parallel, staging assesses the extent of target organ damage, particularly in the cardiovascular, renal, ocular, and nervous systems. Together, this framework enables a comprehensive assessment of disease status, identification of vulnerable or high-risk phenotypes, and implementation of risk-adapted management strategies. Clinically, it facilitates personalized care, promotes collaborative coordination, and strengthens physician-patient communication. Furthermore, this framework provides a scalable structure for integrating disease severity into both individual- and population-level interventions. Although the current criteria for grading and staging are based on expert consensus and selected clinical indicators, such as low C-peptide levels and advanced complications, further validation and refinement are needed. In conclusion, the grading and staging system provides an operational tool for classifying the severity of diabetes mellitus and has the potential to extend life expectancy and improve quality of life for people living with diabetes mellitus.