Three experiments on the comprehension and acquisition of temporal terms are described. Methodological innovations were applied to control for possible methodological effects on children's performance. Each experiment involved 24 children aged 3 to 5. In the first experiment, subjects manipulated toys in response to instructions containing the words "before" or "after." Due to methodological adjustments, the children displayed unprecedented understanding of the temporal terms. The second study used a game against a clock to determine the acquisition order of "before" and "while/when." In response to experimenters' questions, the younger children preferred to use the concept of simultaneity while the older children preferred the concept of precedence. The results also demonstrated that most children have mastered the temporal term "before" by age five. The third experiment demonstrated that most children acquired the term "after" by age five. The results provide evidence of early mastery of both comprehension and production of temporal terms and support for learnability theory and the innateness hypothesis of language acquisition. (RW)