There is a need for non-invasive tests of gastrointestinal and nutritional function. Clinical problems peculiar to infancy and childhood require prompt diagnosis, and methods that are invasive or involve the use of radioisotopes are often impractical or ethically unacceptable. What the pediatrician and clinical scientist seek are tests that are simple, repeatable, and unequivocal in their result for diagnosis, to assess the effects of treatment, and to measure the development of gastrointestinal function during early life. Stable isotope breath tests offer a ready and attractive answer to these needs. They involve the ingestion of substrates labeled with the non-radioactive isotope of carbon (13C), followed by the collection of serial breath samples for analysis of the enrichment of 13CO2, the end product of substrate metabolism. Their non-invasive nature recommends them for use in infancy and childhood, and they can be performed in the ward, clinic, laboratory, and home. In this article I discuss to what degree stable isotope breath tests fulfill the pediatrician's and scientist's needs. I have chosen two examples from the work of myself and my colleagues to illustrate the principles and use of 13C breath tests to detect Helicobacter pylori infection and to measure fat digestion in infancy and childhood.