Several biologic markers correlate with aging and frailty and may offer a better assessment of physiologic age. In combination with geriatric assessment, the use of biologic markers may better be able to personalize oncologic treatments in older adults. This chapter describes various biomarkers of frailty. The inflammatory markers are high levels of white blood cells, C-reactive protein, CXC chemokine ligand 10, interleukin-6, tumor necrosis factor-Td1, fibrinogen, factor VIII, d-dimers, and soluble vascular cell adhesion molecule. Endocrine biomarkers are impaired glucose tolerance, high cortisol level, and low levels of dehydroepiandrosterone sulfate and insulin-like growth factor-1. Telomere shortening and high levels of P16 ink4a are cellular senescence biomarkers. Sarcopenia is the loss of muscle mass associated with aging. Methods to assess muscle mass include whole-body dual-energy x-ray absorptiometry (four limbs) and CT cross-sectional imaging (lumbar muscles).