The significance of predation by protists in aquatic microbial food webs is highlighted in this review. Phagotrophic protists, which feed by engulfing particles, are crucial in aquatic ecosystems, yet they remain understudied compared to bacteria and phytoplankton. These protists range in size from 2 μm flagellates to >100 μm ciliates and dinoflagellates, and exhibit various feeding strategies, including strict phagotrophy, mixotrophy, and autotrophy. They are a major source of mortality for both heterotrophic and autotrophic bacteria, and compete with herbivorous zooplankton for phytoplankton. Protist grazing can influence the rate of organic sinking flux from the euphotic zone, and their excretions contribute to nutrient and trace metal remineralization. Recent advances in methodology, such as molecular genetic analysis and flow cytometry, are aiding research on protist predation. New research areas include the impact of protist predation on prey community structure and chemical communication in microbial food webs. Phagotrophy is an ancient mode of nutrition for protists, present in most major phylogenetic groups, except diatoms and fungi. Some protist groups are strictly phagotrophic, while others are mixotrophic or autotrophic. Protists are widespread and abundant in aquatic ecosystems, playing key roles in food webs, especially in extreme habitats like sea ice, solar salterns, and the deep ocean. Their size is similar to their microbial prey, and their high metabolic rate facilitates carbon and energy flux through ecosystems. In pelagic ecosystems, protists are dominated by heterotrophic flagellates, ciliates, and dinoflagellates. Protist predation is a significant source of mortality for bacteria, and protists can also consume other protists and small zooplankton. Excretion of nitrogen and phosphorus compounds is an important nutrient cycle component. Overall, protists are essential in the functioning of aquatic ecosystems, influencing nutrient cycling, food web dynamics, and ecosystem-level processes.The significance of predation by protists in aquatic microbial food webs is highlighted in this review. Phagotrophic protists, which feed by engulfing particles, are crucial in aquatic ecosystems, yet they remain understudied compared to bacteria and phytoplankton. These protists range in size from 2 μm flagellates to >100 μm ciliates and dinoflagellates, and exhibit various feeding strategies, including strict phagotrophy, mixotrophy, and autotrophy. They are a major source of mortality for both heterotrophic and autotrophic bacteria, and compete with herbivorous zooplankton for phytoplankton. Protist grazing can influence the rate of organic sinking flux from the euphotic zone, and their excretions contribute to nutrient and trace metal remineralization. Recent advances in methodology, such as molecular genetic analysis and flow cytometry, are aiding research on protist predation. New research areas include the impact of protist predation on prey community structure and chemical communication in microbial food webs. Phagotrophy is an ancient mode of nutrition for protists, present in most major phylogenetic groups, except diatoms and fungi. Some protist groups are strictly phagotrophic, while others are mixotrophic or autotrophic. Protists are widespread and abundant in aquatic ecosystems, playing key roles in food webs, especially in extreme habitats like sea ice, solar salterns, and the deep ocean. Their size is similar to their microbial prey, and their high metabolic rate facilitates carbon and energy flux through ecosystems. In pelagic ecosystems, protists are dominated by heterotrophic flagellates, ciliates, and dinoflagellates. Protist predation is a significant source of mortality for bacteria, and protists can also consume other protists and small zooplankton. Excretion of nitrogen and phosphorus compounds is an important nutrient cycle component. Overall, protists are essential in the functioning of aquatic ecosystems, influencing nutrient cycling, food web dynamics, and ecosystem-level processes.