Fibrosis is a major factor in the progressive loss of renal function in patients with a wide variety of kidney diseases. Interstitial fibrosis has a major role in the progression of renal diseases. It is widely recognized that progressive renal disease is accompanied by tubulointerstitial changes characterized by tubular atrophy, increased number of interstitial fibroblasts, phenotypic change of interstitial cells, accumulation of matrix proteins, and interstitial infiltrate of mononuclear cells. Several animal models are available for the study of renal fibrosis. A model of renal fibrosis that encompasses many aspects of other models of kidney disease is unilateral ureteral obstruction (UUO). UUO has emerged as an important model for the study of the mechanisms of renal fibrosis and also for the evaluation of the impact of potential therapeutic approaches to renal disease. Many quantifiable pathophysiological events occur over the span of 1 wk of UUO, making this an attractive model for study. There are many readily quantifiable cellular and molecular events during the initiation and progression of renal fibrosis that make UUO an increasingly good experimental model for study.