Stress or electric field-induced phase transitions have been studied in single crystals of Cr-Cl boracite, Cd₂Nb₂O₇ and Pb(Mg_1/3Nb_2/3O₃ by means of polarized light microscopy associated with electric characterizations. A mm2 phase was found to be induced in Cr-Cl by spontaneous stress at the mechanically non-matching 43m/42m interface at 264 K. Application of an electric field to the 42m phase can induce a mm2 phase, as wa characterized by an antiferroelectric behaviour. In the Cd₂Nb₂O₇ crystals a field-induced phase ttransition mm2(pp)->mm2(ss)was observed. A macroscopically polar phase can be induced in PMN by E, with appearance of macrodomains, birefringent on (100)_cub and (110)_cub and optically isotropic on (111)_cub (O.A//<111>_cub ). the symmetry of the induced phase was deduced to be trigonal 3m. The polarized light microscopy was shown to be a powerful tool for studying the induced phase transitions in ferroic materials.