  We present the relation between the Floquet scattering matrix and the
non-equilibrium Green's function formalisms to transport theory in
noninteracting electronic systems in contact to reservoirs and driven by
time-periodic fields. We present a translation formula that expresses the
Floquet scattering matrix in terms of a Fourier transform of the retarded
Green's function. We prove that such representation satisfies the fundamental
identities of tran sport theory. We also present the ``adiabatic''
approximation to the dc-current in the language of the Keldysh formalism.
