Abstract: Extratropical cyclone is one of the important synoptic systems which affect climate/weather variability over a wide area of mid- and high latitudes, and it is meaningful to research its climatic characteristics, activity patterns, and variation tendency. A comprehensive review of research progress of extratropical cyclones over the Northern Hemisphere from four aspects, involving identification methods, changes during present climate period, changes estimated under the future climate scenarios, and uncertainty analysis of extratropical cyclones, is discussed: (1) In view of the development of reanalysis data and GCM, the research method of extratropical cyclones shifts from subjective analysis to objective identification; (2) There are two main activity centers and two subordinate activity centers displayed by reanalysis and model data, with the main activity center located in the North Pacific and North Atlantic region, and the subordinate activity center located in Mediterranean and Mongolian regions; (3) Under the influence of climate change scenarios by human activities, extratropical cyclones will diminish in most parts of the Northern Hemisphere, and increase in individual districts; (4) Different thresholds, different physical quantities, and considerations of different atmospheric vertical levels add to a picture which can be combined to get an uncertainty view of cyclones, their variability, and trends, in the real world and in GCM studies. Hence, the urgent need for an in-depth understanding of cyclones is more specific intercomparison studies which can demonstrate the differences between the approaches and their results.