Abstract <p>Dinoflagellates are a large group of protists with diverse morphogenetic forms and nutrition modes (phototrophy, mixotrophy, and heterotrophy). Outbreaks of dinoflagellate growth, known as harmful algal blooms (HABs), can cause deaths of fish, birds, mammals, and other animals and pose a significant danger to human health due to the consumption of seafood contaminated with phycotoxins. Therefore, identifying the factors that influence the synthesis of phycotoxins is important for predicting possible HAB events. Increased synthesis of phycotoxins by dinoflagellates under laboratory conditions is necessary for their use in biotechnological applications. This review summarizes the literature data on the main physicochemical and biotic factors influencing the synthesis of toxins by marine photosynthetic dinoflagellates, particularly saxitoxin and its derivatives. The primary focus is on abiotic factors such as nitrogen, phosphorus, and micronutrient concentrations, temperature, illumination, salinity, pH, and the carbon dioxide level. Several biotic factors are also considered. The significant effect of copepodamides, which are low-molecular-weight compounds synthesized by copepods, on the phycotoxin synthesis is noted. The genetic basis for phycotoxin synthesis is discussed using saxitoxin as an example.</p>