Maturity onset diabetes of youth (MODY) occurs in children, adolescents and young adults as a non–insulin-requiring form of diabetes mellitus that is inherited as an autosomal dominant trait. Maturity onset diabetes of youth in whites presents subtly similar to type 2 diabetes in adults. In contrast, a MODY variant that occurs in young blacks, termed atypical diabetes mellitus, presents as an acute-onset form of diabetes. Months to years after diagnosis, atypical diabetes mellitus reverts to a noninsulin requiring course similar to MODY in whites. Five molecular causes for MODY have been identified: mutations in four transcription factors and mutations in one enzyme (glucokinase). Transcription factors regulate gene expression within cells. Mutations in hepatocyte nuclear factor-4α, hepatocyte nuclear factor-1α, insulin promoter factor-1 and hepatocyte nuclear factor-1β, respectively, cause MODY1, MODY3, MODY4, and MODY5. Glucokinase is the glucosensor of the β cell. MODY2 is caused by glucokinase mutations. Although testing for MODY mutations is only available in research laboratories, a careful history and review of the patient’s clinical course can often allow the clinician to diagnose MODY. The diagnosis of MODY has implications for the clinical management of the patient’s diabetes.