The various types of diabetes mellitus are now more precisely classified according to their etiopathogenies. Therefore, type 1 (autoimmune or not), type 2, gestational diabetes, as well as MODY (Maturity Onset-Diabetes of the Young) and secondary diabetes are considered as separate entities. Any type of diabetes brings its set of complications. Biological chemistry is directed to define the etiopathogenic characteristics of the various types of diabetes and to assess the severity of metabolic and functional disturbances. Diagnostic criteria in term of glycemia have also been reviewed to asses diabetes and glucose intolerance.