In this study, recent results from our electron, X-ray, and neutron-diffraction experiments with emphasis on the binary Bi1/2Na1/2TiO3-BaTiO3 (BNT-BT) and ternary Bi1/2Na1/2TiO3-BaTiO3-K0.5Na0.5NbO3 (BNT-BT-KNN) system are presented and contrasted with literature. The experimental results clearly revealed a phase coexistence on the nanoscale level. A systematic study of superlattice reflections in conjunction with microstructural characteristics showed that the BNT-based systems have specific properties in common, which, however, strongly depend on composition. In situ transmission electron microscopy (TEM) electric field experiments unequivocally demonstrated the evolution of lamellar domains. Combining in situ TEM results with published in situ neutron-diffraction experiments, we proposed an electric field-induced phase transition that results in the giant unipolar and bipolar strain observed in specific compositions of the ternary system.