The theoretical work at the interface between analytical and numerical-relativity has played a crucial role in the recent discovery of GW150914 by (i) assessing high detection confidence of the first gravitational-wave passing through the Earth, (ii) unveiling the properties of the binary black-hole merger and (iii) providing us with the most convincing evidence to date that black holes in our Universe are the objects predicted by Einstein’s theory of General Relativity. In this talk I will review the recent discovery and the theoretical work that has paved the way to observe GW150914. I will then discuss the next theoretical challenges to take full advantage of upcoming gravitational-wave observations. (host: S.Rosswog)