Primary intrinsic obstruction to the outlet of the stomach in the neonate is a rare phenomenon, and in the previous edition of this book only five cases had been identified in the preceding 20 year period at the regional neonatal centre in Liverpool. The numbers are of course far outweighed by those children presenting with what is the most common surgically correctable cause of vomiting in the first few weeks of life—infantile hypertrophic pyloric stenosis (IHPS). The justification for including it in a textbook on neonatal surgery is two-fold. Not only do a percentage of the cases present within the true neonatal period, but given the numbers that present to regional neonatal units for management it is often the fledgling Paediatric Surgical trainee’s first exposure to the skills required to successfully manage neonatal cases, both in diagnostic and in operative management terms. This chapter will mainly focus on the current state of knowledge around IHPS; its aetiology, diagnosis and management, before concluding with comments around the pathologies leading to intrinsic obstruction.