  We present a formalism for computing classically measurable quantities
directly from on-shell quantum scattering amplitudes. We discuss the
ingredients needed for obtaining the classical result, and show how to set up
the calculation to derive the result efficiently. We do this without
specializing to a specific theory. We study in detail two examples in
electrodynamics: the momentum transfer in spinless scattering to
next-to-leading order, and the momentum radiated to leading order.
