Marine toxins need a continuous monitoring system that is usually implemented in most producer and consumer countries. This is needed to protect consumers, as marine toxins are very toxic compounds. Up to now, the mouse bioassay has been the method of choice, but there is a legal requirement in Europe to replace the bioassay for mass spectrometry with chromatographic separation. This method is rather complex and expensive, and therefore biosensors are an option that provides an easy, reliable, and affordable fast system. A biosensor commonly used for marine toxin developments is the optical, label free, surface plasmon resonance (SPR). We provide in the chapter a discussion about SPR and a comparison among several technologies, and also a detailed example of an SPR-based method for saxitoxin and analogs. We also provide a method that uses another approach, the resonant mirror, in this case for yessotoxin. Both resonant mirror and SPR are good options for monitoring purposes for marine toxins.