Restoring a functional β cell mass in diabetes patients by β cell transplantation or stimulation of β cell regeneration are promising approaches. It requires knowledge on the mechanisms of β cell neogenesis, an issue that is still quite controversial. Postnatal islet regeneration may or may not depend on an influx of new islet cells from adult progenitors. To solve this issue in animal models, genetic lineage tracing has become a crucial research method. This method allows to test the various hypotheses that have been proposed concerning β cell neogenesis and regeneration.