The incidence of type 2 diabetes mellitus (T2DM) is increasing in youth, largely in correlation with an increase in childhood overweight and obesity. Youth-onset T2DM is a major public health concern worldwide, and tends to show more aggressive features than adult-onset T2DM. Early diagnosis and treatment are important to prevent the occurrence of complications and comorbidities. However, current treatment options are limited and only modestly successful in youth-onset T2DM. Over the last few decades, significant progress has been made in the understanding of youth-onset T2DM. This review summarizes the current understanding of the pathogenesis, diagnosis, and treatment of T2DM in youth. (Ewha Med J 2022;45(3):e3)