Pyloric stenosis, specifically hypertrophic pyloric stenosis, is a condition in infants (typically 2 to 8 weeks old) in which the pylorus — the muscular valve at the outlet of the stomach — becomes abnormally thickened, obstructing the passage of food from the stomach to the small intestine. The classic presentation is forceful, projectile, non-bilious vomiting after feeding. Ultrasound is the definitive diagnostic imaging tool for pyloric stenosis. The key measurements are pyloric muscle thickness (abnormal if greater than 3 millimeters) and pyloric channel length (abnormal if greater than 15 to 17 millimeters). On ultrasound, the thickened muscle appears as a hypoechoic ring surrounding the echogenic mucosa, often described as a “target” or “donut” sign in transverse view. The diagnosis prompts surgical treatment with a pyloromyotomy.