It is well established that YBa 2 Cu 3 O 7 [ m i n u s ] x bulk crystals twin when crossing the tetragonal to orthorhombic transition. Much fewer investigations have been performed on thin films, but one expects a strong influence of the strain induced by the underlying substrate. An analysis of this influence has been undertaken by X-ray diffraction and transmission electron microscopy, from a set of specific samples grown by metal-organic chemical vapor deposition: YBa 2 Cu 3 O 7 [ m i n u s ] x , PrBa 2 Cu 3 O 7 [ m i n u s ] x and YBa 2 Cu 3 O(7[minus ]x)-PrBa 2 Cu 3 O 7 [ m i n u s ] x thin films and superlattices. Results indicate that thick ([gt ] 100 nm) YBCO) films on cubic SrTiO 3 (001) substrates exhibit the same twinning behaviour as bulk crystals. For YBCO films grown on orthorhombic YAlO 3 substrates only one twinning direction is observed. YBCO-PBCO superlattices on SrTiO 3 exhibit an orthorhombic symmetry down to superperiods of 25 [angst ]. The superlattices twinning behaviour is also highly dependent on the total thickness of the stacking.