Teeth are one of the most fascinating hard tissues, studied in biology and paleontology. If their general external morphology provides a general idea about the diet of the animal, their internal structure can also help to understand biology, growth dynamic and ecology of the animals. Their widespread used in paleontology and geochemistry, sometimes preferred over bones, can be explained by a better preservation post-fossilization. I will present three examples of my research. (1) The first one concerns the structure and biological function of the pigmented tooth enamel in shrew teeth. (2) I will also show the different crystallographic and chemical modifications of sauropod teeth enamel and dentine from two different geological settings. (3) In a final part, I will present my last results on histology of “last-toothed” birds from the Cretaceous, from which their inner structure, mode of implantation and attachment tissue gives us new idea about their paleobiology.