A scalar diffraction treatment of forward-angle laser scattering diagnostics (e.g. scintillation interferometry, far-forward scattering) is presented. Results obtained by other workers for Gaussian beam scattering from plasma waves are generalized for arbitrary weakly scattering media and for both near-field (imaging) and far-field (focal plane) experimental configurations. Essential elements of the theory have been confirmed by near-field experimental measurements on airborne ultrasound.