The coherent production of pions at 17·2 GeV in pion interactions with nuclei has been studied using Nuclear Emulsion Technique. The kinematical selection of such events was made out of three prong events in which all the prongs were identified as pions. The mean free path of coherent events was 65 meters. It was found that diffraction dissociation takes place with lighter nuclei, whereas the Coulomb dissociation plays a major role in the case of heavier emulsion nuclei.