Shoaling behaviour in teleosts is crucial for survival, primarily due to the presence of predators and food. Fish shoals exhibit synchronized cooperation to avoid predators and optimize food gathering, balancing the costs and benefits of group membership. Vigilance is essential when predators are present, as survival takes precedence over feeding. Historically, fish schools were seen as leaderless, cooperative societies, but modern ethology shows that social behavior is driven by individual benefit. Fish use strategies like sneaking or scrounging, which are evolutionarily stable. Shoals are not homogeneous; individuals constantly reassess the costs and benefits of being in a group. Teleosts face constraints like swimming speed, affecting their tendency to break ranks. Shoaling functions have been debated, but recent studies challenge earlier views that focused on cover-seeking or hydrodynamics. This chapter reviews current theories and evidence, arguing against the idea that shoaling is mainly for attack avoidance or dilution. Definitions of shoaling and schooling are clarified, with 'shoal' referring to a social group, while 'school' refers to synchronized, polarized groups. Schools are often formed when shoals travel. In North America, 'school' is used for both, but the distinction between facultative and obligate schooling is less emphasized. Figure 12.1 illustrates the relationship between shoaling and schooling behaviors.Shoaling behaviour in teleosts is crucial for survival, primarily due to the presence of predators and food. Fish shoals exhibit synchronized cooperation to avoid predators and optimize food gathering, balancing the costs and benefits of group membership. Vigilance is essential when predators are present, as survival takes precedence over feeding. Historically, fish schools were seen as leaderless, cooperative societies, but modern ethology shows that social behavior is driven by individual benefit. Fish use strategies like sneaking or scrounging, which are evolutionarily stable. Shoals are not homogeneous; individuals constantly reassess the costs and benefits of being in a group. Teleosts face constraints like swimming speed, affecting their tendency to break ranks. Shoaling functions have been debated, but recent studies challenge earlier views that focused on cover-seeking or hydrodynamics. This chapter reviews current theories and evidence, arguing against the idea that shoaling is mainly for attack avoidance or dilution. Definitions of shoaling and schooling are clarified, with 'shoal' referring to a social group, while 'school' refers to synchronized, polarized groups. Schools are often formed when shoals travel. In North America, 'school' is used for both, but the distinction between facultative and obligate schooling is less emphasized. Figure 12.1 illustrates the relationship between shoaling and schooling behaviors.