Older, more vulnerable individuals are increasingly often described in the literature as being frail. Because frailty is often perceived as being undesirable and associated with high health risks, it is important to establish how we can predict, recognize, and treat frailty. Frailty is predisposed by advancing age in combination with physiological deterioration, especially a loss of muscle mass and bone density. Although the symptoms of frailty are diverse, the most common symptoms are a deterioration of activities of daily living (ADL), mobility, nutritional status, cognition, and endurance. The consequences of frailty are institutionalization, morbidity, and mortality. The main determinants of frailty are limitations in ADL, weight loss, diminished mobility or patterns of activity, lowered serum cholesterol level, and sensitivity to change. There is no gold standard for the measurement of frailty, and often studies use a combination of instruments. Although a couple of multidimensional instruments have been developed to measure frailty in its totality, the reliability and validity of these instruments have yet to be established. Successful interventions against frailty include increasing muscle strength through training and individualized recommendations made on the basis of an extensive geriatric assessment.