A sliding hiatus hernia disrupts both the anatomy and physiology of the normal antireflux mechanism. It reduces lower oesophageal sphincter length and pressure, and impairs the augmenting effects of the diaphragmatic crus. It is associated with decreased oesophageal peristalsis, increases the cross-sectional area of the oesophago-gastric junction, and acts as a reservoir allowing reflux from the hernia sac into the oesophagus during swallowing. The overall effect is that of increased oesophageal acid exposure. The presence of a hiatus hernia is associated with symptoms of gastro-oesophageal reflux, increased prevalence and severity of reflux oesophagitis, as well as Barrett's oesophagus and oesophageal adenocarcinoma. The efficacy of treatment with proton pump inhibitors is reduced. Our view on the significance of the sliding hiatus hernia in gastro-oesophageal reflux disease has changed enormously in recent decades. It was initially thought that a hiatus hernia had to be present for reflux oesophagitis to occur. Subsequently, the hiatus hernia was considered an incidental finding of little consequence. We now appreciate that the hiatus hernia has major patho-physiological effects favouring gastro-oesophageal reflux and hence contributing to oesophageal mucosal injury, particularly in patients with severe gastro-oesophageal reflux disease.