Taking tests enhances learning. But what happens when one cannot answer a test question--does an unsuccessful retrieval attempt impede future learning or enhance it? The authors examined this question using materials that ensured that retrieval attempts would be unsuccessful. In Experiments 1 and 2, participants were asked fictional general-knowledge questions (e.g., "What peace treaty ended the Calumet War?"). In Experiments 3-6, participants were shown a cue word (e.g., "whale") and were asked to guess a weak associate (e.g., "mammal"); the rare trials on which participants guessed the correct response were excluded from the analyses. In the test condition, participants attempted to answer the question before being shown the answer; in the "read-only" condition, the question and answer were presented together. Unsuccessful retrieval attempts enhanced learning with both types of materials. These results demonstrate that retrieval attempts enhance future learning; they also suggest that taking challenging tests--instead of avoiding errors--may be one key to effective learning. (Contains 4 figures.)