This chapter reviews sensor-based methods of marine toxin analysis with particular emphasis on immunosensing approaches and their applications. Episodes of marine toxin contamination, produced by phytoplankton, occur worldwide causing both animal and human fatalities. The mouse bioassay is the reference method in current legislation for the majority of marine toxins. Bio-panning is used for the selection of binders from an antibody library that may contain between 107 and 1010 different antibody-encoding gene sequences. Biosensors have the potential to fulfill increasing demands for certification and traceability of seafood products. Biosensor technologies include transduction platforms based on electrochemical, piezoelectric, thermal, or optical methods. Immunosensors can be applied to the detection of small molecular weight toxins produced by toxic marine microalgae. Chromatographic techniques have been widely employed for the sensitive detection of toxins produced by marine microalgae. Optics-based immunosensors for toxin detection, particularly those that incorporate surface plasmon resonance, are widely used.