  We develop systematically to all orders the forward scattering description
for retarded amplitudes in field theories at zero temperature. Subsequently,
through the application of the thermal operator, we establish the forward
scattering description at finite temperature. We argue that, beyond providing a
graphical relation between the zero temperature and the finite temperature
amplitudes, this method is calculationally quite useful. As an example, we
derive the important features of the one loop retarded gluon self-energy in the
hard thermal loop approximation from the corresponding properties of the zero
temperature amplitude.
