Portal hypertension is most commonly associated as a complication related to liver cirrhosis. Portal hypertension increases morbidity and mortality usually through gastrointestinal bleeding however other complications such as ascites formation and subsequent spontaneous bacterial peritonitis may occur. The degree of portal hypertension is already well established as a useful predictor of prognosis. Knowing this information will also help guide management. Currently treatment of portal hypertension is almost empirical without an easily accessible and safe method of monitoring portal pressure response and often it is unclear whether treatment targets have been attained. It is akin to treating arterial hypertension without a sphygmomanometer. Currently the gold standard and most frequently employed method of measuring portal hypertension is via the transjugular method at interventional radiology. This provides a measure of the free and wedged (or occluded) hepatic venous pressure from which the hepatic venous pressure gradient is derived. This gradient is currently the key value to determining risk of morbidity, mortality and guide management, in particular, when to provide active intervention or to help identify non responders to medical therapy. However this is not readily available (only in select expert centres) as this is a long and very technically demanding procedure to produce reliable and consistent clinical data. Therefore the need for an equally safe yet simpler procedure is needed. Endoscopic ultrasound guided portal pressure measurement is a novel technique whereby patients have their portal pressure gradient or hepatic venous pressure gradient measured with direct access into the portal vein and hepatic vein to calculate the hepatic venous pressure gradient. This procedure can be integrated into a session of diagnostic gastroscopy which is already a necessary component of the current management algorithm of portal hypertension. The gastroscopy can identify subjective endoscopic evidence of portal hypertension such as oesophageal varices and the endoscopic ultrasound guided portal pressure measurement can objectively quantify the degree of portal hypertension. In this thesis the author presents the novel technique and results in animal as well as the first human pilot study which has shown great promise and has paved the way for a largerinternational multicentre trial to be commenced in the near future.