Cognitive control enables humans to flexibly switch between different thoughts and actions. An important prerequisite for this cognitive flexibility is the human ability to form and apply general task rules. In this article, I review research investigating the functional role of task rules, with an emphasis on two main findings. First, the shielding function of task rules helps guide attention toward task-related information, thereby reducing possible distraction by irrelevant information. Second, this task shielding has to be relaxed when a task rule changes, thereby making the cognitive system more vulnerable to the intrusion of distracting information. Implications for developmental psychology and higher-level cognition are discussed.