Conventional liver tests can be used to estimate a mixture of injury and function but none of these may be regarded as a reliable marker either to quantify functional hepatic reserve or to reflect life-threatening complications of acute and chronic liver diseases. To overcome this limit, many dynamic tests have been developed in order to evaluate the "hepatic functional mass". Among these tests we can include breath tests with 13C-labeled substrates undergoing different metabolic pathways. As concerning the evaluation of microsomal function, two main categories of breath tests have been developed based on the limiting step in the different substrates metabolism. The first group include aminopyrine, caffeine and diazepam, all substrates with a metabolism independent from hepatic blood flow and dependent almost exclusively from the enzymatic activity of different cytochromes P450. The other group is composed of substrates with flow dependent metabolism like methacetin, phenacetin, erythromycin. The aim of this review is to describe the clinical applications of microsomal liver breath tests in different hepatic diseases.