Non invasive evaluation of gastric emptying is generally performed by scintigraphy which is, however, difficult to perform and not suitable to children and childbearing women. A new method based on stable isotope breath testing analysis has been introduced in clinical practice: the 13C-octanoic acid breath test. In this paper, an overview of the current knowledge on this technique is given with special emphasis on the principle of the test, the mathematics used to analyse the results, and the physiological, pathological, and pharmacological aspects of gastric emptying studied with this new method.