This thesis describes a series of studies undertaken to explore the criteria for the stabilization of Bi3+ in a perovskite through the formation of mixed A-site solid solutions. Electronically similar to Pb2+ but with a smaller ionic radius, Bi3+ has the potential to replace and improve upon Pb-based counterparts for dielectric and piezoelectric applications. Building off the discovery that Bi(Zn1/2Ti1/2)O3 increases the tetragonality of PbTiO3, the first part of this work identified a new substituent, Bi(Zn3/4W1/4)O6, that is also able to increase the c/a of PbTiO3. In the process, the crystal chemical criterion for enhancing the tetragonality of PbTiO3 was clarified. From there, these tetragonality-enhancing additives were introduced into lead-free perovskite chemistries in an attempt to stabilize a new generation of environmentally benign ferroelectric materials. The final section returns to lead-containing systems by investigating ternary systems in an attempt to capture the high c/a of the mixed Pb-Bi systems at an MPB. Unexpected dielectric behavior was observed in some compositions, which was shown to be the result of the structure entering a region of 2-phase coexistence. A new crystal-chemical parameter is defined to describe the variance of the B-site displacement factors and used to develop an empirical model for predicting the incidence of this 2-phase coexistence region. An atomistic model similar to those of relaxor ferroelectrics was proposed to explain the behavior.