Microscopic planktonic algae are the main food source for filter-feeding bivalves. However, among the secondary algal metabolites, several have been identified as potent toxins that can have negative effects on human health and environment (okadaic acid, domoic acid, saxitoxins, brevetoxins). Biotoxins associated with human intoxication are product of several species of dinoflagellates, diatoms and blue-green algae. The variety of shellfish species used in human nutrition can accumulate phytotoxins in their edible tissue. By consumption of shellfish with high concentration of accumulated biotoxins, people can develop different symptoms of intoxication, based on which we distinguish four serious poisoning syndromes. These are diarrhetic shellfish poisoning (DSP), paralytic shellfish poisoning (PSP), amnesic shellfish poisoning (ASP) and neurotoxic shellfish poisoning (NSP). Because of possible serious effect on human health and environment, number of countries conducts continuous algal monitoring and assays for accumulated biotoxins detection in shellfish. Over the past few decades reports of blooms of potentially toxic algae and appearance of human intoxication have become more frequent, and it seems that the increase in anthropological eutrophication of water bodies is the main cause.