Aging is an inevitable, irreversible, and complex biological process that negatively impacts multiple organ systems and their ability to function. The aging of the central nervous system makes the patient more prone to confusion, the aging of the renal system to dehydration or drug-related toxicity. Genetic, epigenetic, and environmental factors affect this process. This chapter reviews age-associated physiologic changes according to different systems: cardiovascular system, respiratory system, central nervous system, renal system, hematopoietic and immune systems, gastrointestinal system, genitourinary system, musculoskeletal system, skin system, and sensory system. Study of aging is difficult, primarily because it is a highly heterogeneous phenomenon with inter- and intra individual variability. Comprehensive assessment of elderly patients is of the highest importance, as some elderly patients may be vulnerable and cancer or the therapy may decompensate an underlying frailty.