In 1934, Whitney asked how can we determine whether a jet of order m on a closed subset E of Rn is a jet of order m of a C m function. It is well-known that the answer is such jets must be Whitney fields. Later, Paw lucki and Kurdyka proved that subanalytic Whitney fields of order m are restrictions of subanalytic C m functions. Here, we discuss Whitney’s Extension Theorem in o-minimal structures (joint work with Matthias Aschenbrenner).