Diabetes mellitus (DM), the most common endocrine disease, is a metabolic syndrome characterized by hyperglycemia due to absolute or relative insulin deficiency/resistance or both. Diabetes mellitus is classified according to etiopathogenesis that leads to hyperglycemia, into different groups, but majority of cases fall into one of two broad classes; namely type 1 DM and type 2 DM. In the majority of patients, immunologic markers appear before diabetes becomes clinically overt. Beta cell mass and insulin secretion progressively decreases although normal glucose tolerance is maintained. Written in question and answer format, this chapter talks about the clinical features and diagnosis of DM, complications of diabetes, and chronic complications of diabetes. Crisp and to the point answers of questions have been given in this chapter.