The gastroesophageal junction is structurally complex and functionally designed to ensure the acid secreted by the most proximal gastric mucosa flows towards the stomach and not up onto the oesophageal squamous mucosa. The pattern and mechanism of reflux vary with the severity of reflux disease and this probably represents different ends of a spectrum rather than distinct pathophysiological mechanisms. Nearly all patients with severe reflux disease have hiatus hernia, however, a substantial proportion of patients with mild reflux disease do not, and this may be a result of intermittent or partial hiatus hernia undetectable by current available tools. The acid pocket is an area of post-prandial unbuffered gastric acidity immediately distal to the gastroesophageal junction and which is enlarged in patients with hiatus hernia. The acid pocket provides a reservoir of acid available to reflux when the intrinsic sphincter fails. Central obesity is an important factor in the aetiology of reflux and does this by the increased abdomino-thoracic pressure gradient inducing hiatus hernia and increasing the rate of flow of reflux when sphincter opens. Central obesity also induces short segment intrasphincteric reflux and thereby columnar metaplasia of the most distal oesophagus.