Dinoflagellates are microscopic, mostly unicellular algae that live in freshwater and marine waters. Only a relatively small number of Dinoflagellate species produce bioactive molecules that are toxic to other organisms. This chapter discusses the marine dinoflagellates known to produce toxins, especially those species that produce toxins that accumulate through food chains to cause human poisoning. In addition to dinoflagellates, a number of freshwater and brackish species of cyanobacteria are known to produce the toxins. The majority of paralytic Shellfish poisoning contamination of Shellfish around the world is caused by species of the planktonic Dinoflagellate belonging to the genus Alexandrium. Neurotoxic Shellfish poisoning is a neurological disease caused by eating Shellfish contaminated with lipid-soluble, polyether toxins called brevetoxins. Gambierdiscus spp. are benthic dinoflagellates that are mostly found as epiphytes in oligotrophic waters on a range of biotic and abiotic substrates including macroalgae, turf algae, detritus, and sand.