The structure function (or the power spectrum)of phase fluctuations induced by atmospheric turbulence is the main quantity which must be known to optimize the High Angular Resolution techniques used for astronomical observations. It allows to evaluate the state of coherence of the perturbed wavefronts. In this paper are firstly recalled classical results concerning the propagation of the luminous waves through the terrestrial atmosphere and consequently the image formation. Then, different models generally used to describe the optical effects of atmospheric turbulence, due to numerous thin layers, are presented and their influence discussed. A second order statistical analysis is performed and the coherence parameters deduced. The necessity of measuring these parameters appears evident and dedicated monitors must be built.