Ecological interactions between individuals in populations are often dependent on the relative sizes of the individuals. Because of this, interactions within and between size-structured populations can have complex dynamics. The dynamics of size- structured populations will be studied using several species of salamanders that coexist in Appalachian streamside habitats. These salamanders grow through a wide range of sizes during their lives, and because all are generalized predators, there is great potential for competitive, predatory, and other behavioral interactions between size classes within and among species. Previous work has shown that some of these interactions between size classes are strong and are likely to be important in determining community structure. Field and laboratory experiments will explore these interactions to determine their mechanisms. These experiments will aid in developing models of the dynamics and stability of size-structured populations.