Vesicoureteral reflux (VUR) is a condition in which urine flows retrograde, or backward, from the bladder into the ureters/kidneys. Urine normally travels in one direction (forward, or antegrade) from the kidneys to the bladder via the ureters, with a 1-way valve at the ureterovesical (ureteral-bladder) junction preventing backflow. The valve is formed by oblique tunneling of the distal ureter through the wall of the bladder, creating a short length of ureter (1–2 cm) that can be compressed as the bladder fills. Reflux occurs if the ureter enters the bladder without sufficient tunneling, i.e., too "end-on."