Diabetes mellitus is a group of diseases characterized by chronic increase of glucose level. The last years brought progress in understanding the multiplicity of its forms, as well as, its complex pathogenesis. In 1999, a classification of diabetes based on the etiology of individual types, was proposed by the Experts Committee of the World Health Organization, and is now commonly accepted. Etiologic classification in the last decade was gradually extended with the progress of knowledge, in particular, with successes of researchers in the field of genetics. Monogenic forms of diabetes such as MODY, mitochondrial diabetes, neonatal diabetes and lipoatrophic diabetes, discovered over a dozen years ago, are characterized by unique clinical features and possibility of applying a tailored treatment, assuring optimal correction of genetically conditioned metabolic defect. The differential diagnostics of types of the diseases is playing an increasing role in diabetology, as it enables selection of optimal treatment methods, as well as, the assessment of prognosis referring to the diabetes course and complications occurence. In this article, a review of problems associated with the differential diagnostics of diabetes and its practical clinical application was made.