  Motivated by connections with observable phenomena, in particular with soft
factorization theorems for scattering amplitudes and with memory effects,
renewed interest has been recently shown in the subject of asymptotic
symmetries at null infinity. The two main goals of this Ph.D. thesis are,
first, to review the main aspects of the connection between such symmetries and
observable effects in the context of gravity, electromagnetic and Yang-Mills
theory in four dimensions and, second, to present results concerning the
extension of this program to the case of spacetimes of arbitrary dimension,
either even or odd, to the emission or absorption of soft scalar quanta, in
connection with their dual description, and to theories containing massless
higher-spin fields.
