This article gives an overview of the role of sliding hiatus hernia in gastro-oesophageal reflux disease (GORD). The crural diaphragm acts as an external sphincter of the anti-reflux barrier. Contractions of the crural diaphragm increase lower-oesophageal-sphincter (LOS) pressure during each inspiration and in situations of increased abdominal pressure, whereas these contractions are inhibited when gas and/or a bolus has to pass the gastro-oesophageal junction. A hiatus hernia is associated with GORD symptoms, increased oesophageal acid exposure, and oesophagitis and its severity. In patients with hiatus hernia, the incidence of reflux episodes is increased during periods with low LOS pressure, straining and swallowing. These findings underline the importance of the crural diaphragm, which, when surrounding the LOS, protects against gastro-oesophageal reflux.