The Ideal Free Distribution (IFD) is a foundational model originally developed to describe the foraging behavior of groups of organisms as a function of resource availability. According to the IFD, individuals are predicted to optimize resource intake by foraging proportionally to resource input, thereby maximizing obtained reinforcers in the context of a group. As an equilibrium model, the IFD is especially powerful in that systematic deviations from its predictions help identify factors influencing decision-making in groups. In this chapter, we trace the development of IFD through behavioral ecology, highlighting conceptual roots in behavioral economics and equilibrium-based models. We then review empirical tests of the IFD in both human and nonhuman populations, emphasizing some deviations that revealed limitations of the original model. Finally, we discuss refinements of the IFD that incorporate factors such as competitive interference, limited information of alternatives, travel costs, and resource dynamics, and extensions of the model to different areas of study. Through these examples, we illustrate how the IFD serves as both a predictive model and a tool for identifying variables shaping group decision-making.