Patients’ skeletal muscle mass plays an important role in physical fitness, quality of life and the ability to recover from disease. During the past centuries, focus on the role of physical fitness, adequate nutrition, and psychological wellbeing as parts of the total concept of health has increased. Multiple factors, such as the introduction of ERAS programs, growth of minimally invasive surgery, centralization of health care and mandatory clinical auditing have enhanced successful outcomes of surgical care. However, while focus was laid on improvement of processes and structural factors, we forgot to include “the patient” in these optimization programs. Therefore, the aim of this thesis was to describes the dynamic changes of skeletal muscle mass in the perioperative period, and to use muscle mass as a factor to identify several patient-specific leads in the perioperative period which could be used to improve surgical healthcare even further. In the preoperative phase, assessment of skeletal muscle mass can help to identify frail patients who could benefit from prehabilitation prior to surgery. And during the postoperative phase, insufficient nutritional intake and physical activity seem to be important risk factors for developing severe postoperative muscle loss. Since this thesis shows that skeletal muscle loss in the first week after surgery is associated with worse one-year survival, all clinicians, patients, and current medicine students should be aware of the importance of preservation of skeletal muscle mass and the prevention of perioperative muscle loss.