In this narrative review we describe the relationships between muscle mass and surgical outcomes in regards to protein calorie malnutrition (PCM), frailty, and sarcopenia, to guide future research. Studley first demonstrated a correlation between body composition and surgical outcomes in 1936. Studley demonstrated that whilst contemporary surgical risk factors did not correlate with outcomes, there was a clear correlation with severe pre-operative weight loss. Resulting research demonstrated weight loss alone was an unreliable predictor unless accompanied by physiological impairment. Later PCM assessments focused on body weight, body mass index, serum albumin, and other biochemical measures, but these markers were confounded by inflammation, fluid shifts, and acute illness. A critical advance demonstrated that arm muscle circumference and grip strength correlated with post-operative morbidity, whereas weight, weight-loss and serum albumin levels did not. These findings were attributed to skeletal muscle acting as a key metabolic reserve, providing amino acids and energy for protein production, wound healing and recovery. Frailty syndrome was established as a marker of biological age. Some meta-analyses demonstrated no relation between age and surgical outcomes once frailty was accounted for. Frailty is a strong predictor of surgical morbidity across specialties, however measurement of frailty is complex. Several meta-analyses comparing the effectiveness of individual frailty domains revealed that sarcopenia and reduced muscle strength account for much of frailty’s prognostic power. Although contemporary sarcopenia definitions increasingly emphasise both muscle mass and strength, evidence supporting their superiority over muscle mass alone remains limited. Advances in imaging now allow accurate assessment of muscle mass, however various measures are in use and the most effective prognostic measure has not been established. Further research investigating this is needed.