Algicides have a role in management strategies for toxic cyanobacteria (blue-green algae). When used correctly they have the attraction of terminating the problem at the source in the reservoir, and this is a "once-off", if treatment is successful. The algicide of choice is copper sulphate. Copper sulphate has been used widely to control algal blooms in water supply storages and lakes for nearly 100 years. It is generally regarded as effective, economical and safe for operators to use, although copper can have adverse environmental impacts on the aquatic ecosystem. Other popular algicides include a range of copper chelates. The effectiveness of copper-algicide treatment is determined by a combination of chemical, physical and biological factors. Chemical factors include the characteristics of the receiving water. Factors such as the pH, alkalinity and dissolved organic carbon all determine copper speciation and complexation, which reduces the toxicity of copper in solution. Physical factors, particularly thermal stratification in the reservoir, affect the distribution of copper after application, which in turn may determine contact with the target organism. The important biological factor is the sensitivity of the target organism to copper. Cyanobacteria are generally regarded as being relatively sensitive to copper toxicity. The important point to remember about copper is that it is a broad-spectrum aquatic biocide, which will affect non-target species such as zooplankton and fish, and this can have significant adverse environmental effects. Local environmental regulations may determine the conditions under which algicides can be used. There are several important issues to be aware of when treating toxic cyanobacteria with algicides. Firstly, they should be applied at the early stages of bloom development when cell numbers are low. This will reduce the potential for the release of high concentrations of intracellular toxins associated with dense blooms. Algicides will disrupt healthy cyanobacterial cells leading to the release of toxins and taste and odor compounds into solution. These dissolved toxins will then disperse and be diluted throughout the water body. Secondly, the lysis of cells by algicides compromises the effectiveness of toxin removal by conventional filtration. Intracellular toxins contained in intact cells are relatively well removed by flocculation and filtration steps in the treatment process. This can lead to reliance on adsorption by activated carbon or advanced oxidation to achieve effective removal of dissolved toxins from treated water. Thirdly, a withholding period may be required after algicide treatment to allow toxins in the reservoir to dilute, disperse and degrade. Includes 91 references, tables, figures.