The embryonic insult that results in unilateral renal agenesis may involve not only the ureteral bud but also other mesonephric duct derivatives, including the seminal vesicles, vas deferens, and epididymis; in the female with a solitary kidney, müllerian duct anomalies frequently occur. Normal renal development depends upon a normal ureteral bud, which undergoes orderly branching and penetrates the metanephric blastema at about the fifth week of gestation. Ureteral and kidney development are thought to be interdependent, and when there is failure of the ureteral bud to form or absence of the nephrogenic ridge, the kidney does not develop normally. Unilateral renal agenesis is compatible with normal longevity and does not predispose the contralateral kidney to greater-than-normal risk; nevertheless, patients should have annual surveillance, including a blood pressure measurement, serum creatinine if not initially normal, and urinalysis to detect proteinuria. Removal of one kidney leads to structural and functional changes by the remaining kidney, including increased filtration of the remaining glomeruli. These functional changes have generally been considered beneficial because they mitigate the reduction in the total glomerular filtration rate that would otherwise occur, but experimental evidence suggests that these changes may have an adverse effect on the remaining kidney. Clinical evidence shows that these changes do not lead to renal deterioration in kidney donors because the renal function of kidney donors is well preserved in over 20 years of follow-up after donor nephrectomy.