Summary: Maturity-Onset Diabetes of the Young (MODY) is a rare type of diabetes. It is a monogenic disease associated with abnormalities in 14 genes defined so far (HNF1A, HNF4A, GCK, PDX1, HNF1B, NEUROD1, KLF11, CEL, PAX4, INS, BLK, ABCC8, KCNJ11, WFS). This diagnosis is based on the results of genetic testing. The clinical picture is very similar to type 1 diabetes, less often type 2 diabetes, therefore the delay in the genetic diagnosis of MODY is estimated at about 15 years. One useful diagnostic tool is an online MODY probability calculator, which can give a probability score of a patient having MODY. It can be used in patients under 35 years of age. The correct diagnosis of MODY is associated with the optimization of treatment (metformin vs. sulfonylurea vs. insulin therapy), supervision over possible complications of the disease and the ability to conduct genetic counseling.