The chapter discusses the functions of shoaling behavior in teleosts, emphasizing the roles of predators and food. Shoaling behavior is crucial for both defense against predators and optimal foraging. The text highlights that while fish schools are often seen as egalitarian leaderless societies, contemporary ethology reveals that social behavior is more complex, with individuals constantly reevaluating the costs and benefits of being social. The chapter also addresses the limitations of previous theories, such as those by Hamilton, Williams, and Breder, which have incorrectly associated shoaling primarily with cover-seeking or hydrodynamics. It introduces a new taxonomy that distinguishes between "shoals" (social groups) and "schools" (synchronized and polarized swimming groups), providing a more nuanced understanding of fish behavior. The chapter aims to review the evidence for the functions of shoaling behavior in light of current theories, highlighting the underlying tensions and decisions that individuals make regarding their social interactions.The chapter discusses the functions of shoaling behavior in teleosts, emphasizing the roles of predators and food. Shoaling behavior is crucial for both defense against predators and optimal foraging. The text highlights that while fish schools are often seen as egalitarian leaderless societies, contemporary ethology reveals that social behavior is more complex, with individuals constantly reevaluating the costs and benefits of being social. The chapter also addresses the limitations of previous theories, such as those by Hamilton, Williams, and Breder, which have incorrectly associated shoaling primarily with cover-seeking or hydrodynamics. It introduces a new taxonomy that distinguishes between "shoals" (social groups) and "schools" (synchronized and polarized swimming groups), providing a more nuanced understanding of fish behavior. The chapter aims to review the evidence for the functions of shoaling behavior in light of current theories, highlighting the underlying tensions and decisions that individuals make regarding their social interactions.