A red tide is a type of harmful algal bloom that occurs when certain types of algae grow out of control. Algae are tiny, microscopic organisms that live in the water, and almost all bodies of water have some algae. However, in a red tide, there is a lot more algae in the water than usual, and the water changes color because the population of algae living in the water becomes so dense. The name "red tide" comes from the fact that overgrowth of algae can cause the color of the water to turn red, as well as green or brown.

Red tides have been around since long before humans, but certain human activities are making them more frequent. Chemicals from farming, factories, sewage treatment plants, and other sources can become dissolved in water on the land. This water, called runoff, eventually flows into the ocean and can cause algae to grow faster, leading to red tides.

Red tides are sometimes also called harmful algal blooms (HABs). Some of the algae that cause a red tide produce powerful toxins, which are harmful chemicals that can kill fish, shellfish, mammals, and birds. If people eat fish or shellfish that have been in the water with toxic algae, they will also ingest the toxins, which can make them sick. Many regions restrict fishing during a red tide for this reason. Nearby restaurants take local fish and shellfish off the menu, too.

Red tides can also cause respiratory irritation in people who inhale the toxins, and they can make the surrounding air difficult to breathe. Coastal communities that rely on tourism lose millions of dollars when dead fish wash up on beaches or beachgoers experience eye and respiratory irritation, and shellfish-harvesting businesses lose income when shellfish beds are closed.