Type 1 diabetes mellitus (T1DM) is classically described as absolute insulin deficiency secondary to autoimmune destruction of pancreatic β-cells. It affects mostly children and young adults. Usually, the disease presents with osmotic symptoms, weight loss, and/or diabetic ketoacidosis. The latter condition may sometimes be life-threatening. Individuals having T1DM with positive family history of obesity and type 2 diabetes mellitus (T2DM) are at increased risk of being overweight and obese. Obesity, insulin resistance (IR), altered adipocytokine profile, and altered lipid handling in individuals with T1DM may attenuate the vascular benefit of glycemic control in long run, predisposing them to future micro- and macrovascular complications. Double diabetes, estimated glucose disposal and cardiovascular disease, metabolic syndrome in type 1 diabetes mellitus, type 1 diabetes mellitus and lipid handling, type 1 diabetes mellitus, proinflammatory cytokine, and metabolic syndrome, etc. some other important topic of this chapter, which are also covered in this chapter.