The occurrences of harmful blooms in Danish coastal waters during the past20 years have been analysed. Harmful algal blooms are a recurrent phenomenonin Danish marine waters. The blooms develop in the saline waters off thewestern coast of Jutland (=eastern North Sea and Skagerrak) and in the"inner coastal water" (Kattegat and Belt Sea = brackish waters between theNorth Sea and Baltic Sea). The harmful blooms may be divided into two groups: the regular blooms andthe exceptional blooms. Regular blooming species are Prorocentrum minimum,Gymnodinium mikimotoi, and Nodularia spumigena. Among the most well knownexceptional blooms are the Chrysochromulina polylepis bloom in 1988 and theChattonella sp. blooms in 1998. The harmful effects of the blooms vary fromnot detectable (Prorocentrum minimum) to mass death of bottom fauna andother fauna/flora (Gymnodinium, Chrysochromulina). Common for blooms is thatthey are often followed by oxygen depletion. In general the frequency ofblooms has not increased (or decreased).To improve the forecast of blooms a number of large scaled blooms have beenanalysed. Biological, chemical and hydrographic data on the blooms arecollected by the Danish counties and the National Environmental ResearchInstitute. In addition, meteorological data and satellite image are used forthe analyses. Using case stories the most important factors inducing bloomsin Danish marine waters will be discussed.Not surprisingly, the decisive factors that can be use in prognosticationare weather conditions, i.e. wind, irradiance, precipitation and derivativeparameters as mixing, currents, photon fluxes and nutrients. Danish marinewaters are however complex due to the location in the transition zonebetween the very brackish Baltic Sea and the marine North Sea. Comprehensiveknowledge of cause-effect relations is thus needed to make operationalmodels that can predict blooms. The potential of making a prognostic modelfor the Danish marine waters is demonstrated.