A U-shaped relationship between body mass index (BMI) and outcomes emerged after cardiac surgery. This review analyses the physio pathologic basis of obesity related complications and evaluates prognostic implications. Both leaner and morbid obese should be considered pre-operatively rather than reactively and, when referred for elective surgery, should undergo a focused metabolic status management, and a thorough evaluation of health status. Adherence to sound surgical principles, and tailored patient blood management and perioperative care are mandatory.