Individual-based ecology (IBE) refers to theoretical and applied ecology conducted in ways that recognize that individuals are important. IBE addresses how the dynamics of populations, communities, and ecosystems are affected by characteristics and behaviors of the individual organisms that make up these systems. The main tools of IBE are individual-based models (IBMs), simulation models that represent ecological systems as collections of explicit individuals and their environment. Theory in IBE consists of models of individual characteristics—especially adaptive behaviors—that have proven useful for explaining system-level phenomena, by testing them against empirical information. This kind of theory is unique in two ways. First, it is across-scales: the aim is not to merely seek good models of individual behavior but models of individual behaviors that explain system dynamics. Second, theory is readily tested against many kinds of observations so that its defi nition more closely resembles that of theory in physical sciences: models that are “general” in the sense of solving many real-world problems.