This document chronicles the discovery of the Higgs boson and early measurements in the diphoton decay channel. Particular attention is paid to photon identification, to the coupling of the Higgs to the vector bosons, and to differential cross sections of the Higgs boson. As these measurements yielded good agreement to the predictions of the Standard Model, an additional search is performed, for Higgs boson pair production in the γγbb final state. The dataset used represents 5/fb of proton- proton collisions at sqrt(s) = 7 TeV and 20/fb of collisions at sqrt(s) = 8 TeV, recorded by the ATLAS experiment at CERN's Large Hadron Collider in 2011 and 2012.