  On-shell scattering amplitudes have proven to be useful tools for tackling
the two-body problem in general relativity. This thesis outlines how to compute
relevant classical observables that are themselves on-shell, directly from
amplitudes; examples considered are the momentum impulse, total radiated
momentum, and angular impulse for spinning particles. As applications we derive
results relevant for black hole physics, computing in the post-Minkowskian
expansion of GR, and construct a worldsheet effective action for the leading
spin interactions of Kerr black holes.
