AbstractPhysiological and ecological aspects of the application of copper sulfate as an algicide within aquatic ecosystems are reviewed. Excessive developments of algae due to eutrophication of stagnant waters have to be controlled by preventive measures preferably. Application of algicides should be an exception. The mostly used algicide is copper sulfate because it is effective, selective, economical and undesired ecological side‐effects could not be detected. Since blue‐green algae are very copper sensitive, blooms of that species can be controlled effectively. Although toxicity to fish and invertebrates has been proved for a wide range of concentrations, they are actually not endangered by the commonly applied copper concentrations, because they show avoidance reactions and are only affected by prolonged exposition to copper. Some authors, however, showed free ions to be quickly removed from the water body. Organic compounds, water hardness and high alkalinity decrease the amounts of soluble and toxic copper specimens. Some blue‐green algae species produce strong copper‐complexing and detoxifying agents‐extracellular polypeptides and hydroxamic acids. Some chelatic agents, on the other hand, increase the algicidal effectivity of copper in alkaline waters. Also lipid‐soluble ligands increase copper toxicity in many cases. Some uncertainty in knowledge call for further research.–to determine the very right moment for copper application before algal bloom appearance to avoid oxygen depletion and fish kills‐therefore to improve forecaste of bloom development.–to define the dependence of copper toxicity on the physiological state of the algae.–to search for the effects of co‐stress (simultaneous pH‐shock) in order to restrict copper concentration to low levels and–to estimate and forecast partitioning of copper to the extracellular (chemically bound) and intracellular (taken