Feedback can be a powerful learning tool, but its effects vary widely. Research has suggested that learners' prior knowledge may moderate the effects of feedback; however, no causal link has been established. In Experiment 1, we randomly assigned elementary school children (N = 108) to a condition based on a crossing of 2 factors: induced strategy knowledge (yes vs. no) and immediate, verification feedback (present vs. absent). Feedback had positive effects for children who were not taught a correct strategy, but negative effects for children with induced knowledge of a correct strategy. In Experiment 2, we induced strategy knowledge in all children (N = 101) and randomly assigned them to 1 of 3 conditions: no feedback, immediate correct-answer feedback, or summative correct-answer feedback. Again, feedback had negative effects relative to no feedback. Results provide evidence for a causal role of prior knowledge and indicate that minimal feedback can both help and hinder learning.