Stable isotope methodologies offer a number of possibilities for the nutritional assessment of many different processes and metabolic pathways. The application of stable isotopes has been boosted by the development of new mass spectrometers, lower costs of probes, and the risks associated with radioactive tracers. The use of 13C as a tracer offers all the advantages of stable isotopes and has been widely applied for measuring various types of metabolic processes. This review is focused on clinical and nutritional assessments using 13C breath tests.