In this paper, existing domain-switching criteria are briefly reviewed and the predictions based on these domain-switching criteria are compared with the available experimental data for 180degrees and 90degrees domain switchings in lead lanthanum zirconate titanate (PLZT) and lead zirconate titanate (PZT), respectively. It is found that the predictions do not match the experimental results. Motivated by this observation, a new domain-switching criterion in terms of internal-energy density is proposed for combined mechanical and electrical loads. Based on consideration of the atomic structure of ferroelectric materials, it is recognized that 180degrees and 90degrees domain switchings result from different mechanisms and, thus, require different critical internal-energy densities for switching. The new criterion is found to yield very good predictions for both 180degrees and 90degrees domain switchings.