After Haro’s fundamental discovery of flare stars in stellar associations and young clusters, their importance was fully recognized. The flare star system provides one of the most important records of the stellar aggregate’s history. This record can be used to establish the chronology of these systems and to test theories of star and aggregate formation.
Unfortunately, the observational material contains an unavoidable strong selection with respect to the statistics and the physical characteristics of flare stars in aggregates. This explains the difficulties which arise when one tries to compare physical and statistical characteristics of flare stars in aggregates and in the solar neighborhood, since the latter objects are studied almost exclusively by photoelectric methods. The flare frequency in aggregates is more than one order of magnitude smaller than that of flare stars in the solar neighbourhood. This is probably a selection effect, since the photographic observations in aggregates have been carried out with exposure times 5-10 min. Small amplitude flares cannot be recorded at all. Flare stars in aggregates are usually objects whose recorded flares have amplitudes > 0m.6 − 0m.7, and which last for at least 5 min. The long exposure masks the true amplitude of the flare.