Communications with coherent light through the atmosphere bring out new system limitations besides the ones imposed by attenuation and scattering normally associated with incoherent waves. This is so because the laser directivity and coherence responsible for increased channel capacity are easily destroyed or at least degraded by the presence of random thermal turbulences. Simple expressions describing the effects of atmospheric turbulence on laser beam propagation are obtained. Corresponding calculations yield the order of magnitude of such random phenomena as beam scanning, phase variation, beam cross-section change, amplitude and frequency modulation. In particular, the polarization fluctuation is predicted quantitatively in terms of the atmosphere mean square refractive index and its turbulence correlation length.