  In the weak field regime, gravitational waves can be considered as being made
up of collisionless, relativistic tensor modes that travel along null geodesics
of the perturbed background metric. We work in this geometric optics picture to
calculate the anisotropies in gravitational wave backgrounds resulting from
astrophysical and cosmological sources. Our formalism yields expressions for
the angular power spectrum of the anisotropies. We show how the anisotropies
are sourced by intrinsic, Doppler, Sachs-Wolfe, and Integrated Sachs-Wolfe
terms in analogy with Cosmic Microwave Background photons.
