In this talk I will first review the basics of asymptotic symmetry algebras of (flat) spacetimes, the BMS algebras, focusing on the 4 dimensional case. I will then give a (non-exhaustive) overview of the recent developments on the connection between BMS algebras , the gravitational S-matrix and the so-called 'memory effect' . For the 3 dimensional case,I will show that an ultra-relativistic, flat, Inonu-Wigner contraction of the Virasoro algebra gives rise to the BMS algebra, and hence argue its relevance in the field of flat-holography. I will then show how this procedure can be exploited to obtain all supersymmetric extensions of the BMS_3 algebra, some of which are completely novel and possess peculiar energy bounds.
