Technical Abstract: Significant losses occur every year in aquaculture, farm livestock, and waterfowl due to problems with toxin and "off-flavor" compound production from microalgae blooms. In addition, an overabundance of algae can clog streams and create flavor problems for municipal drinking water supplies. At present, application of synthetic compounds to the affected aquatic ecosystem is one management method used to control and prevent the growth of noxious phytoplankton. Unfortunately, many of these synthetic compounds have limitations for their usefulness in controlling phytoplankton blooms including restricted use (by government), broad-spectrum toxicity towards non- target organisms, high toxicity to non-target organisms, and public perception of adverse health risks associated with their use. The discovery, characterization, and use of natural compounds for phytoplankton control would provide environmentally safe alternatives to chemical compounds. This chapter focuses on past and present research in natural algicides including isolation and characterization of bioactive lead compounds, toxic selectivity and mechanism of action of lead natural compounds (and materials), and efficacy in aquatic ecosystems in terms of impact on target phytoplankton blooms and non-target organisms. The economic cost of using natural algicides is also discussed.