The role of prior knowledge in retrieval of Spanish-English vocabulary pairs learned using keyword mediators was examined in 4 experiments. Retrieval was tested immediately after learning and after 1-week and 1-month no-practice intervals (Experiment 1), after moderate retrieval practice (Experiment 2), and after extended retrieval practice (Experiments 3 and 4). Using accuracy, latency, and verbal report data, a detailed account of memory retrieval processes was developed. Initial retrieval is an explicit mediation process that involves retrieving keyword mediators into working memory and using them as retrieval cues to access the English equivalents of the Spanish words. After extended vocabulary retrieval practice, this sequential mediation process qualitatively changed to a direct retrieval process in which the English equivalent was accessed in a single working memory step. However, direct retrieval was still influenced by a covert mediation process.