The eukaryotic picoplankton is a heterogeneous collection of small protists 1 to 3 μm in size populating surface oceans at abundances of 102 to 104 cells ml−1. Pigmented cells are important primary producers that are at the base of food webs. Colorless cells are mostly bacterivores and play key roles in channeling bacteria to higher trophic levels as well as in nutrient recycling. Mixotrophy and parasitism are relevant but less investigated trophic paths. Molecular surveys of picoeukaryotes have unveiled a large phylogenetic diversity and new lineages, and it is critical to understand the ecological and evolutionary significance of this large and novel diversity. A main goal is to assess how individuals are organized in taxonomic units and how they participate in ecological processes. Picoeukaryotes are convincingly integral members of marine ecosystems in terms of cell abundance, biomass, activity, and diversity and they play crucial roles in food webs and biogeochemical cycles.