  Searches for new phenomena in high-mass diphoton final states with the ATLAS
experiment at the LHC are presented. The analysis is based on $pp$ collision
data corresponding to an integrated luminosity of 36.7 fb$^{-1}$ at a
centre-of-mass energy $\sqrt{s}=13$ TeV recorded in 2015 and 2016. Searches are
performed for resonances with spin 0, as predicted by theories with an extended
Higgs sector, and for resonances with spin 2, using a warped extra-dimension
model as a benchmark model, as well as for non-resonant signals, assuming a
large extra-dimension scenario. No significant deviation from the Standard
Model is observed. Upper limits are placed on the production cross section
times branching ratio to two photons as a function of the resonance mass. In
addition, lower limits are set on the ultraviolet cutoff scale in the large
extra-dimensions model.
