Type 2 Diabetes Mellitus (T2DM) is a noncommunicable, lifestyle-related chronic metabolic disorder of global involvement, characterized by elevated blood sugar levels, manifested by hyperglycaemia, polyuria, polydipsia and polyphagia. DM is associated with acute and chronic complications which lead to reduced quality of life, premature morbidity and mortality. T2DM is linked with overweight, obesity, reduced physical activity and a genetic component. T2DM is named a silent killer because the primary disease is silent at the early stage and usually gets diagnosed when presenting with a vascular event such as stroke or heart attack. Impaired cardiorespiratory fitness plays a crucial role in acceleration of cardiovascular complications resulting in premature organ damage, morbidity and mortality. Regular physical activity, resistance training and reduction in sedentary life style along with diet control and drugs help to control DM and prevent or delay complications. This chapter deals with diabetes as a disease, its prevalence, risk factors, signs and symptoms, pathophysiology, pathogenesis and underlying mechanisms, acute and chronic complications, along with measures to enhance cardiorespiratory fitness and control DM and a word of caution to the younger generation to be aware of the silent killer.