A phycotoxin is a toxic compound produced by algae, particularly marine dinoflagellates and diatoms, that accumulates in filter-feeding shellfish and finfish and poisons humans who eat them. The family includes saxitoxin, causing paralytic shellfish poisoning; domoic acid, causing amnesic shellfish poisoning; okadaic acid, causing diarrhetic shellfish poisoning; and ciguatoxins in reef fish. Harmful algal blooms, the “red tides,” trigger monitoring programmes that close harvesting areas when toxin levels exceed limits, since cooking does not destroy these heat-stable compounds. Climate change is expanding the range and frequency of toxic blooms, raising the stakes for surveillance. Phycotoxins are natural toxins in the strictest sense, produced without any human pollution, and they make shellfish safety one of the most testing challenges in food regulation.