With the aging of the population and the increase in older survivors of critical illness with physical and cognitive impairments, there is a need to better identify critically ill adults at high risk of adverse outcomes. Frailty, a syndrome of age- or disease-related declines in physical or cognitive reserve, is a useful construct for identifying critically ill older adults with diminished physiologic reserve. This seminar will 1) review the biology of aging and frailty; 2) summarize frailty assessment tools that can be used in the intensive care unit (ICU) setting and 3) discuss specific strategies for leveraging these tools for improving the care of the elderly patient in the ICU.