An assemblage consists of the various species populations of a larger taxon in a defined area. Assemblage structure refers mainly to the number of individuals, species, and families, and the predator–prey interactions and other trophic relationships between fishes (e.g., Matthews 1998). Ecological interactions thus occur within a fish assemblage, within a spiderassemblage, etc. Focusing on assemblages is admittedly myopic, since fishes interact with invertebrate prey and parasites, with plants as food and shelter, with reptiles, birds, and mammals as predators, etc. However, fish–fish interactions are particularly obvious, it is often logistically difficult to deal with all components of an ecosystem, and researchers tend to specialize and develop expertise in certain taxonomic groups (hence the rationale for producing an ichthyology or any other taxon-oriented textbook). For the purposes of the present discussion, we will look at competitive and predator–prey interactions that tend to involve fishes of different species, and discuss prevalent ideas on how assemblages are structured and ordered, i.e., how interactions between fishes affect species composition and maintenance of assemblages.