NASICON (Na₃Zr₂Si₂PO₁₂) has shown promise as a solid state electrolyte for a sodium battery due to its inherent safety and relatively high conductivity.  Increasing the conductivity of NASICON to further improve the capability of a solid state electrolyte has come by way aliovalent substitution of the zirconium site in Na₃Zr₂Si₂PO₁₂. Both Na_3.2Zr_1.8M_0.2Si₂PO₁₂ (M=Al³⁺, Fe³⁺, Y³⁺) and Na_3.4Zr_1.8M_0.2Si₂PO₁₂ (M=Co²⁺, Ni²⁺, Zn²⁺) have exhibited greater bulk conductivity at room temperature than undoped NASICON. A decrease in low temperature activation energy as well as the lowered rhombohedral-monoclinic phase transformation temperature is responsible for the increased bulk conductivity of substituted NASICON.  The Co, Zn, and Ni dopants were explored more extensively as they displayed the highest conductivity of 10% doped NASICON at room temperature. By adjusting the total substitution of Co, Zn, and Ni cations, the conductivity of NASICON was optimized further. Increasing the amount of Zn⁺² dopant in Na_3+2xZr_2-xZn_xSi₂PO₁₂ resulted in reduced distortion of the lattice as it transitioned from rhombohedral to monoclinic phase, and an increase in conductivity. Ultimately the bulk ionic conductivity of 20% Zn-doped NASICON is among the highest ever reported.