Polar snow and firn represents the result of snow metamorphism at very long time scales of many tens of years and in a broad variability of climate conditions. At polar ice sheets the diverse snow types of firn layers and the long term behavior of their microstructure and density with depth and at different sites can be studied. We present microstructure properties of polar snow and firn obtained by X-Ray Micro-Computer-Tomography from various sites in Greenland and Antarctica with different annual mean temperatures and accumulation rates. We will show and discuss the evolution of three-dimensional microstructural features such as coordination number, shape factor or specific surface area with depth and time at the different sites. In order to parameterize the evolution of microstructure with time we map out universal characteristics and regional distinctions of the polar snow- and firn microstructure.