The aqueous chemistry of the Zr-H2O system is reviewed. It is shown that the thermodynamically stable solid phase for the Zr-H2O system at 25°C is the monoclinic phase. After a review of the chemistry of the pure material in water, a summary of the solution equilibria of the Zr-H2O system is presented followed by a discussion of the aqueous phase stability of the monoclinic and tetragonal phases of the pure ZrO2 system. Last, a discussion of the phase stability of the complex metal oxides, particularly the Y2O3-containing TZP in aqueous solution will be reviewed. It will be shown that the phase stability for the complex metal oxide-zirconia systems may be best understood as corrosion with the alloying agent (e.g., Y2O3) subject to leaching from a zirconia-rich interface.