Sixty-four mothers of children ranging in age from 6 to 58 months were asked to determine, for pairings of play and language items, which item was more advanced developmentally. This procedure was repeated within 2 weeks. In general, mothers' orderings of play and language items matched those established in the developmental literature and were stable over the short term. Mothers' knowledge about language development was stronger than and unrelated to their knowledge about play, suggesting that maternal knowledge about developmental domains is differentiated and specific. Finally, mothers' judgments about the developmental milestones depended on their children's current developmental stage: Mothers were less accurate at estimating the timing of milestones that their children had mastered many months earlier, supporting the view that mothers' knowledge is informed by their children's recent rather than past achievements in specific areas.