The paper reports on a study of extremely cool summers (ECS), a rare but significant phenomenon responsible for various adverse economic effects. These include, in particular, effects on agriculture and the tourist industry. The seasons, their spatial extent and individual characteristics were identified in Central and Eastern Europe. The research was based on average seasonal air temperatures (June to August) and the numbers of days with maximum temperatures >25, 30 and 35 ^{\circ}C and minimum temperatures >20 ^{\circ}C, as recorded at 59 weather stations during the period 1951-2010. An ECS was defined as having an average temperature at least 2 standard deviations (t ≤ tav. - 2\sigma) lower than the average at a given station during the study period. ECS occurred at only 34 of the 59 stations (58 % of all stations), one or two per station, mainly in the northern part of the study area. Six such seasons were recorded by at least 5 % of all stations in 1962, 1969, 1976, 1978, 1984 and 1994. The average temperature of an ECS was typically 2-3 ^{\circ}C lower than the long-term average. While some ECSs included an extremely cool month, most such seasons (58 %) were defined by low temperatures persisting for long periods in all summer months.