SummaryFor most surgeons and many anaesthetists, patient frailty is currently the 'elephant in the (operating) room': it is easy to spot, but is often ignored. In this paper, we discuss different approaches to the measurement of frailty and review the evidence regarding the effect of frailty on peri-operative outcomes. We explore the limitations of 'eyeballing' patients to quantify frailty, and consider why the frail older patient, challenged by seemingly minor insults in the postoperative period, may suffer falls or delirium. Frailty represents a state of increased vulnerability to stressors, and older inpatients are exposed to multiple stressors in the peri-operative setting. Quantifying frailty is likely to increase the precision of peri-operative risk assessment. The Frailty Index derived from Comprehensive Geriatric Assessment is a simple and robust way to quantify frailty, but is yet to be systematically investigated in the preoperative setting. Furthermore, the optimal care for frail patients and the reversibility of frailty with prehabilitation are fertile areas for future research. (While I can't define it)…I know it when I see it.-Justice Potter Stewart, US Supreme Court Clinicians usually recognise a frail patient, who is often, but not always, elderly. However, at best, we usually quantify frailty in a largely subjective 'end-of-the-bed' or 'eyeball' way [1]. Furthermore, although studies have examined patient factors such as age and ASA physical status and their association with peri-operative complications and mortality [2-4], until very recently, few have examined frailty [5]. Frailty increases vulnerability to stressors, which, in the peri-operative setting, can include the procedure(s), complications and medical care administered.The aims of this paper are to review the emerging science around defining and quantifying frailty, and to examine its associated peri-operative risks. Current evidence suggests that quantifying frailty improves the precision of risk assessment [5]. However, we think that, for most surgeons and many anaesthetists, frailty and its associated risks represent an 'elephant in the (operating) room': it is easy to spot, but is often ignored.