A theoretical model is presented for the formation and growth of polar stratospheric clouds (PSCs). The process is assumed to occur in three stages: (1) a ''precursor'' stage of supercooled H2SO4H2O stratospheric aerosol droplets at temperatures well above the frost point; (2) an intermediate (type I) stage of PSC particle formation by codeposition of HNO3⋅3H2 at temperatures near, but above the frost point; (3) a final (type II) stage of deposition of pure water ice and HNO3⋅3H2O at temperatures below the frost point. The calculated temperature dependence of optical backscattering agrees well with values observed during two Arctic airborne lidar experiments except for small systematic errors at the 30--mbar pressure level which may be due to poor characterization of the temperature field. Additional theoretical calculations address the issue of PSC formation in Antarctica. These results show that at the 70--mbar level (near 17 km), about 80% and 30% of the respective HNO3 and H2O vapor available may be sequestered in relatively large (4--μm radius) PSC particles at a temperature near 189 K, some 2 K below the estimated local frost point. Such large particles would fall at a rate of about 2 km wk-1s, suggesting the PSCs may act to redistribute these trace gases in the stratosphere or perhaps remove them altogether by sedimentation to the troposphere. ¿ American Geophysical Union 1988