Playtoxin (PLTX) is one of the most potent marine natural toxins so far known, and evidence suggests that species of the genus Ostreopsis are its potential production sources. This chapter focuses on the ecobiology and origin of this fascinating molecule, PLTX, and also highlights the most important up-to-date available data regarding its chemistry. It deals with current knowledge on the ecobiology, origin, and chemistry of naturally occurring PLTX analogs produced from different species of the Dinoflagellate genus Ostreopsis. PLTX is a large and very complex molecule with both lipophilic and hydrophilic areas, possessing the longest chain of continuous carbon atoms known to exist in a natural product. The detection of PLTX or a closely related compound was reported in the red alga Chondria crispus. The chapter provides a detailed summary of the developments regarding chemical analysis methods for this group of compounds, with special reference to their detection and quantification in seafood.