Ten expert authors have combined to provide a comprehensive summary of the status of knowledge, circa 2000, about “polar lows” in both polar regions. The term “polar low” is normally reserved for small but fairly intense maritime cyclones that dominantly form in the northern oceans during winter, as cold air crosses regions of sharp sea surface temperature gradients. This synthesis covers the full spectrum of mesoscale lows with a diameter less than 1000 km that occur in the Arctic and Antarctic poleward of the main polar front. These features typically form and develop in data-sparse areas, so heavy reliance is placed on satellite remote sensing and numerical modeling to describe and understand these storms. Only a small number of systems have been directly sampled by aircraft. A particularly strong and attractive aspect of Polar Lows is the plethora of satellite images that illustrate the wide range of cloud signatures.