Harmful algal blooms (HABs) and their associated toxins are a signicant threat to human health. Cyanobacteria and other HAB organisms produce a variety of toxins, including neurotoxins, hepatotoxins, dermatotoxins, and other bioactive compounds. As discussed in Chapter 1, the most typically cited classes of HAB toxins include microcystins, saxitoxins (STXs), anatoxins, and cylindrospermopsin. Although complete understanding of the health effects of HABs and HAB toxins has not yet been developed, HAB toxins are known to cause a variety of short-and long-term health effects in animals and humans. In natural systems, HABs result in sh kills and may lead to death or acute health effects in animals and livestock. Shortterm acute effects of HAB toxins on humans include rashes, liver inammation, numbness, dermatitis, gastrointestinal problems, liver failure, and others. Documented longer-term impacts associated with low-level chronic exposure to HAB toxins are less well known but potentially include tumor formation, cardiac arrhythmia, and liver failure.