The presence of pectenotoxins and yessotoxins in Shellfish was discovered due to their high acute toxicity in the traditional mouse bioassay after intraperitoneal injection of lipophilic extracts. This chapter describes chemistry, metabolism, and chemical detection methods of pectenotoxins and yessotoxins. Pectenotoxins resemble okadaic acid in molecular weight and in having cyclic ethers and a carboxyl group in the molecule. The origin of pectenotoxins is the toxic Dinoflagellate genus Dinophysis, which is also the causative species for Diarrhetic Shellfish poisons (DSP) toxins. In contrast to pectenotoxins, yessotoxins are relatively stable under strongly basic conditions such as those used to hydrolysis of acyl esters of the okadaic acid groups. Due to the difficulties in distinguishing between DSP toxins and other lipophilic toxins by the traditional mouse bioassay, several chemical analyses have been developed. The origin of pectenotoxins is the toxic Dinoflagellate genus Dinophysis, which is also the causative species for DSP toxins.