Phytoplankton form the base of the food web in aquatic systems. In marine environments, they play important roles in nutrient cycling and oxygen production, which is crucial for the survival of marine organisms. These phytoplankton can produce useful or harmful metabolites, which can impact the surrounding ecosystem, food security as well as human health. In this chapter, we will examine the various metabolites that come from the three major phytoplankton groups that have been identified from the coastal waters of Singapore, namely: cyanobacteria, diatoms and dinoflagellates. Useful metabolites such as photoprotective pigments, including fucoxanthin, scytonemin and β-carotene, would be discussed with respect to its cosmetic applications. Under suitable environmental conditions, these phytoplankton can proliferate rapidly and produce toxic metabolites, which cause harmful bloom events. We highlight key toxins, such as saxitoxins and karlotoxins, which can result in shellfish poisoning upon the consumption of contaminated seafood. We also look at beneficial carbohydrates and proteins that are produced by the various phytoplankton groups and evaluate their industrial applications as biofuels and a source of dietary protein.