Maturity-onset diabetes of the young (MODY) is a non–insulin-dependent form of diabetes mellitus that is usually diagnosed in young adulthood. MODY is most often an autosomal dominant disease and is divided into subtypes (MODY1 to MODY14) based on the causative genetic mutation. Subtypes 1 to 3 account for 95% of cases. MODY is often misdiagnosed as type 1 or 2 diabetes and should be suspected in nonobese patients who have diabetes that was diagnosed at a young age (younger than 30 years) and a strong family history of diabetes. Unlike those with type 1 diabetes, patients with MODY have preserved pancreatic beta-cell function three to five years after diagnosis, as evidenced by detectable serum C-peptide levels with a serum glucose level greater than 144 mg per dL and no laboratory evidence of pancreatic beta-cell autoimmunity. Patients with MODY1 and MODY3 have progressive hyperglycemia and vascular complication rates similar to patients with types 1 and 2 diabetes. Lifestyle modification including a low-carbohydrate diet should be the first-line treatment for MODY1 and MODY3. Sulfonylureas are the preferred pharmacologic therapy based on pathophysiologic reasoning, although clinical trials are lacking. Patients with MODY2 have mild stable fasting hyperglycemia with low risk of diabetes-related complications and generally do not require treatment, except in pregnancy. Pregnant patients with MODY may require insulin therapy and additional fetal monitoring for macrosomia. (Am Fam Physician. 2022;105(2):162–167. Copyright © 2022 American Academy of Family Physicians.)