Renal aging, by itself, is associated with alterations in renal morphology and a decline in renal function, which is accelerated and/or accentuated by diseases such as diabetes mellitus and hypertension. The aging-related renal insufficiency has important implications with regards to body homeostasis, drug toxicity, and renal transplantation. An understanding of renal aging and its distinction from renal insufficiency secondary to diseases is essential for individualized care of the elderly. Toward this end, investigations are underway to elucidate the molecular mechanisms of renal aging. This review summarizes the structural and functional changes of the aging kidney and highlights the advances made in our understanding of the renal aging process.