Superimposed ice is freshwater ice that forms from refreezing of snow meltwater on the sea-ice or slush surface as this interface is always colder than the freshwater freezing point. Its formation, thickness and lifetime strongly depend on the surface energy balance. Here we describe and summarize key processes of superimposed ice formation on sea ice under different atmospheric conditions, as they are characteristic for different regions (e.g. Arctic, Antarctic and lower latitudes). Physical properties (density, grain sizes, salinity, growth rates) are quantified based on field observations and freezer-lab analysis from three expeditions to Arctic and Antarctic sea ice. In addition, a comprehensive summary of hitherto studies on superimposed ice on sea ice is given, and their results are included into our discussion and conclusion. For strong melting, superimposed ice formation results from percolation and refreezing. In contrast, for low snow wetness, compaction and subsidence of snow dominate the superimposed ice formation. The superimposed ice thickens due to grain growth in saturated snow on top of older ice. The texture of superimposed ice consists of polygonal granular crystals. Their size depends on the temperature gradient during the formation process.