The goal of ecology is to understand the distribution and abundance of organisms, generally by quantifying how abiotic conditions and species interactions contribute to population growth. Much ecology focuses on simple pairwise interactions, such as competition and predation; yet, species naturally exist in much more complex systems in which their abundances are determined by webs of species interactions. An important step for ecologists has been to understand how interactions may occur through loops and webs of connected species: it is these interactions that are now loosely collected together into what we call “indirect effects.” Some types of indirect effects are thought not only to widely occur but also to be particularly important for determining both the abundances of individual species and community properties such as diversity and stability. These include trophic cascades, where predators enhance producer growth by feeding on consumer species, and keystone predation, where predators consume dominant competitors, thus allowing inferior competitors to persist. Identifying and quantifying indirect effects has become a major issue in ecology. Although their general importance is well understood, we have little understanding of the relative importance of different types of indirect effects.