Optimality theory can be used to predict the responses of foragering animals to changes in prey density. Predator-prey theory is interested in whether predators cause density-dependent mortality of their prey. These 2 bodies of thought were combined by determining conditions under which optimal foraging results in density-dependent predation. Optimal foraging contributes to density-dependent predation if the optimal behavior (e.g., size of foraging area, length of foraging period, foraging speed, proportion of time spent in risky, rewarding areas) increases with increasing prey density. If the forager''s goal is to maximize net benefit, then for food-limited foragers or those with high foraging costs, the optimal behavior should generally increase with prey density. If foragers are near satiation or if costs also increase with prey density, then the optimal behavior can decrease with increasing prey density. If the goal is to minimize foraging time while attaining a benefit threshold, then the predictions are identical to those for benefit maximizers. If the goal is cost or activity minimization, then the optimal behavior should generally decrease with increasing prey density.