MODY (maturity-onset diabetes of the young) is the most prevalent form of monogenic diabetes, resulting from beta-cell dysfunction. Classically, MODY presents as non-insulin-dependent diabetes before the age of 25–30 years in an autosomal dominant inheritance pattern. Currently, mutations in at least 14 genes are associated with the MODY phenotype. These phenotypes differ in frequency, clinical features, chronic complications, and response to therapy. The most common forms are caused by mutations in the GCK, HNF1A, and HNF4A genes, which account for about 80% of MODY cases. Carriers are often incorrectly classified as type 1 or type 2 diabetes mellitus given some clinical similarities between these different types of diabetes. The correct diagnosis is important because it allows for appropriate therapy and has implications for prognosis and genetic counseling.