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 this book is the plethora of satellite images that illustrate the wide range of cloud signatures. In the introductory Chapter 1, J. Turner, E. Rasmussen, and A. Carleton give a brief history of research, and follow this with discussion of the vexing problem of labeling, for which many different descriptions have been used (polar low, mesoscale cyclone, Arctic instability low, polar air depression, etc.). Satellite images are then presented, showing comma cloud, spiraliform, merry-go-round, instant occlusion, baroclinic wave, and warm core types of mesoscale cyclones. Chapter 2, by Rasmussen, K. Ninomiya, and Carleton, addresses the climatology of mesoscale cyclones in the Arctic and the Antarctic in relation to the physical factors that occur in these regions. For example, the ubiquitous katabatic winds near the Antarctic coastal slopes play a central role in generating low-level frontal zones just offshore that are key to mesoscale cyclogenesis in that area. The spatial and temporal variations of mesoscale cyclones are then related to the large-scale modes of atmospheric variability, such as the North Atlantic Oscillation and the El Ni¿o-Southern Oscillation.