This study examines the conditions under which 3-year-olds can use the desires of others to predict others' behavior. In Study 1, children were highly successful in predicting the actions of an agent based on that agent's desires when they were explicitly told about the agent's desires, even when the agent's desires were strongly different from the children's own. Study 2 showed that 3-year-olds could also predict the actions of an agent when they had to infer the agent's desires from the previous good and bad experiences of the agent and from information about the agent's general behavioral preferences. Studies 3 and 4 demonstrated that children had difficulty predicting an agent's behavior when they both had to infer the desire of the agent and this desire conflicted with their own desires. These results suggest that preschoolers' desire reasoning is sophisticated but also may be influenced by the processing demands of the task. (Contains 4 tables and 1 footnote.)