Interfacial defect theory is applied to the epitaxial ferroelectric system consisting of a tetragonal ferroelectric such as BaTiO3or PbTiO3grown onto a cubic (001) substrate. The interfacial defects that result from the diffusionless paraelectric to ferroelectric (PE→FE) phase transition are treated under the constraint that no misfit dislocations are generated during or as a result of the transition. The domain pattern develops to provide strain relief in the film. The interfacial defects for the ...c/a1/c/a1... domain pattern include coherency edge dislocations and coherency wedge disclinations. Interfacial defects for the ...a1/a2/a1/a2... domain pattern include coherency edge and screw dislocations. Far‐field strain states for both domain patterns can be predicted from the interfacial defect content. From the twinning geometry, general expressions are derived for the far‐field rotations of the crystal axes of individual domains for the ...a1/c/a1/c... and the ...a1/a2/a1/a2... domain pattern. The geometrically predicted rotation angles for ...c/a1/c/a1... domain pattern are verified by x‐ray‐diffraction and transmission electron diffraction for epitaxial PbTiO3films grown on (001) SrTiO3substrates. ©1995 American Institute of Physics.