  In this thesis we present a study of the computation of classical observables
in gauge theories and gravity directly from scattering amplitudes. In
particular, we discuss the direct application of modern amplitude techniques in
the one, and two-body problems for both, scattering and bounded scenarios, and
in both, classical electrodynamics and gravity, with particular emphasis on
spin effects in general, and in four spacetime dimensions. Among these
observables we have the conservative linear impulse and the radiated waveform
in the two-body problem, and the differential cross section for the scattering
of waves off classical spinning compact objects. Implications of classical soft
theorems in the computation of classical radiation are also discussed.
Furthermore, formal aspects of the double copy for massive spinning matter, and
its application in a classical two-body context are considered. Finally, the
relation between the minimal coupling gravitational Compton amplitude and the
scattering of gravitational waves off the Kerr black hole is presented.
