  These notes consist of 3 lectures on celestial holography given at the
Pre-Strings school 2021. We start by reviewing how semiclassically, the
subleading soft graviton theorem implies an enhancement of the Lorentz symmetry
of scattering in four-dimensional asymptotically flat gravity to Virasoro. This
leads to the construction of celestial amplitudes as $\mathcal{S}$-matrices
computed in a basis of boost eigenstates. Both massless and massive asymptotic
states are recast as insertions on the celestial sphere transforming as global
conformal primaries under the Lorentz SL$(2, \mathbb{C})$. We conclude with an
overview of celestial symmetries and the constraints they impose on celestial
scattering.
