Observations of polar stratospheric clouds by the Improved Stratospheric and Mesospheric Sounder (ISAMS) experiment on the Upper Atmospheric Research Satellite (UARS) have revealed new details of their global properties and behavior. These include the vertical and horizontal spatial distributions of Arctic and Antarctic polar stratospheric clouds (PSCs) as a function of time and air temperature, their optical thicknesses and estimated densities, their spectral properties, and their inferred composition. In particular, ISAMS spectral data allows different PSC types to be distinguished from each other and from volcanic aerosol by their compositional differences. Northern PSCs during the 1991/92 season are found to be more ephemeral and more compact than reported in previous years and to differ markedly in scale from those in the Southern Hemisphere, which cause the Antarctic ozone hole by activating stratospheric chlorine chemistry. There were only two episodes of dense PSC formation in the 1991/92 northern winter, one of which took place in sunlight. The latter correlates well with UARS/Microwave Limb Sounder observations of enhanced chlorine monoxide, but substantial amounts of chlorine monoxide were also reported at times and places with at most very minor PSC activity.