High voltage olivine structured cathodes offer the potential for high energy dense storage. LiCoPO₄ (LCP) has a theoretical discharge capacity of 167 mAhg^-1 at 4.8 V. Its adoption has been limited by poor cycling owing to both structural instability of the de-lithiated material and electrolyte instability at 5 V. LiNiPO₄ (LNP) with a similar discharge capacity has a discharge voltage of 5.1 V for an even greater energy storage potential. We have shown that ion substitution is a powerful method to stabilize the cycling of the LCP high voltage electrode material in its charged, fully de-lithiated state. A low level of Fe substitution for Co increases the electrical and ionic conductivity and stabilizes the structure through change in the electronic structure which shift the redox activity toward Fe/Co and away from oxygen. Substitution of Cr for Co further increases the redox activity of Co leading to higher discharge capacity than for Fe-only substituted LCP. This stabilization mechanism is confirmed through spectroscopic measurements, through DFT calculation and is evidenced by extremely reduced discharge capacity fade during electrochemical cycling. Addition of Si to the cathode reduces the reactivity with the electrolyte as evidenced by an increase in the coulombic efficiency. LNP is even more challenging with extremely low electrical conductivity and an unfavorable electronic structure. We will discuss attempts to improve its performance. Electrolyte stability has been addressed through sacrificial additives, changes in solvent composition and surface modification of LCP. This paper will touch upon the most recent results in the development of a stabilized high voltage olivines with high energy and excellent cycle life and the development of supporting electrolytes.