Physical fitness, based on tests of physical performance and measurements of daily activity, can serve as both a risk stratification variable and clinical outcome for patients prior to or during cytotoxic chemotherapy. Many cytotoxic therapies cause premature aging and frailty in a sizable portion of patients, so measuring and improving physical function may also limit late morbidity and mortality in certain patient populations. Clinician-rated performance status (cPS; e.g., ECOG or Karnofsky PS) in addition to organ-specific function have been used to prognosticate risk and determine treatment strategy, but cPS only readily identifies individuals with significant functional compromise. Objective evaluations of physical function, such as aerobic capacity and measured daily activity, may have greater potential to improve the precision of performance status evaluation.