Harmful algal blooms on the U.S. west coast are caused by specific types of algae that produce toxins harmful to shellfish and fish. These toxins can cause shellfish poisoning and fish deaths. Some algae also create red tides, which are not toxic to humans. The presence of algal toxins along the coast has been noted for centuries, but scientific knowledge is limited. The study reviews the causes of these blooms, their dynamics, and the species responsible for them.