Breath tests for "dynamic" liver function evaluation have been proposed several years ago. A variety of carbon-labelled breath tests for the assessment of mitochondrial, microsomal and cytosolic liver function have been described with the aim to increase data on liver disease staging, prognosis, and response to therapy. In the last years a great interest is developed about the use of breath test for liver mitochondrial function evaluation since it results impaired in a wide range of liver diseases either of genetic or acquired origin. In these cases mitochondrial oxidative metabolism of some substrates, as far as recovery of the hepatic energy state after a metabolic insult, results impaired because of the disfunction of the electron transport chain and/or ATP synthesis. Ketoisocaproic acid and methionine are the best studied carbon-labelled substrates for the investigation of mitochondrial functional damages related to structural alterations that occur in many liver diseases. Although these tests are simple, cost-effective and safe, to date there is still not general approval for their usefulness in clinical settings since they should fulfill several requirements to overcome the drawbacks of traditional quantitative tests. On the other hand, this field is relatively young and further studies are needed in order to assess the suitable substrate for the evaluation of the complex mitochondrial metabolism both in healthy subjects and in patients with liver disease.