In this talk, the speaker will present the Lorentz transformations of flat spacetime act as conformal transformations on the celestial sphere. This makes it possible to construct conformal correlators from flat space amplitudes. He will first review the recent progress on this construction through conformal primary wave functions and discuss the implication of unitary and crossing symmetry. Lastly, he will comment on its connection to AdS/CFT (anti-de Sitter/conformal field theory) and flat space holography.