The damage to marine resources caused by red tides represents a serious problem for fisheries and allied industries. In Japan, most fish farms have suffered heavy losses from red tides of the dinophyteGymnodinium nagasakienseand of the raphidophytesChattonella antiqua, C. marina, andHeterosigma akasbiwo. Therefore, an extensive series of studies has been conducted in Japan. Elucidation of the mechanisms of outbreak of harmful red tides associated with fish kills and the development of countermeasures for these red tides are urgently needed. Recently,Heterosigmared tides have caused severe losses for salmon culture operations in several places around the world. The development of fish farming and the progress of eutrophication on a global scale will accelerate the incidence of harmful red tides. This article is a review of the physiological ecology of these organisms and of the techniques proposed inJapanto predict the outbreaks of harmful red tides in areas that have experienced damage from them.