Removal of toxic substances from the blood depends on patent connections between the kidneys, ureters and bladder that are established when the ureter is transposed from its original insertion site in the Wolffian duct, to the bladder trigone, its final insertion site. According to the Ureteric Bud Theory of Mackie and Stephens, this repositioning of the ureter orifice occurs as the trigone forms from the common nephric duct, the caudal-most Wolffian duct segment. The availability of mouse models has enabled us to re-examine this hypothesis in the context of normal and abnormal development. We find than in contrast to what has been previously thought, the common nephric duct does not differentiate into the bladder trigone but instead undergoes apoptosis, a crucial step in ureter transposition. Interestingly, apoptosis only occurs in close proximity with the sinus ridge, a raised epithelial structure located at the dorsal aspect of the urogenital sinus, suggesting that signals from this site may be important for normal ureter insertion. These studies provide new insights into the normal process of ureter maturation, and shed light on possible causes of obstruction and reflux, ureteral abnormalities that affect 1–2% of the human population.