Paralytic shellfish poisoning (PSP) toxins are toxins produced by harmful red tide algae, such as some species of marine dinoflagellate and blue-green algae. PSP toxins, including saxitoxin and its derivatives are tetrahydro purines composed of two guanidinium functions. PSP toxins can selectively block the voltage-gated sodium channel of many excitable membranes. The 7, 8, 9-guanidinium moiety is essential for the blocking activity. PSP toxins can be used in many fields. In the study of red tide, purified PSP toxins can serve as standard toxins to analyze algal and shellfish samples. Development of new method for toxin-monitoring also needs purified PSP toxins. In studies of neurobiology and molecular biology, saxitoxin has been widely used as research tool. PSP toxins also show activities as local anesthetics, in treating cancer and so on. However, the functions still need to be confirmed and studied. In summary, PSP toxins have great potential for different applications despite the huge damages they have made in the past several decades.