When talking to children, mothers around the world use infant-directed speech (IDS), a speaking style characterized by exaggerated prosody as compared to adult-directed speech (ADS). This dissertation investigates the role of prosodic input in children’s word learning by focusing on IDS in word-learning contexts in which mothers introduce unfamiliar words to their children. This research took a cross-linguistic approach to examine IDS in two typologically distinct languages: Dutch and Mandarin Chinese. A semi-spontaneous storybook-telling task was used to elicit both ADS and IDS from mothers. Also, a word-learning experiment was conducted to evaluate children’s online word learning performances. Three key findings emerged. First, both Dutch and Chinese mothers are sensitive to their children’s vocabulary knowledge and adapt their speech prosody accordingly in word-learning contexts. However, IDS prosody in word-learning contexts is manifested differently across languages. Second, the prosody of Dutch IDS specific to word-learning contexts is correlated with children’s vocabulary size and vocabulary growth. Third, Dutch 24-month-old children can reliably learn novel words from both ADS and IDS, but IDS has a small facilitative effect on children’s online word learning compared to ADS. In conclusion, prosodic input in word-learning contexts is fine-tuned for linguistic purposes and plays a significant role in children’s word learning. This dissertation may be of interest to researchers working on infant-directed speech, prosody, and early language acquisition. More broadly, it may be of interest to early childhood educators as well as caregivers who are interested in children’s language development in the first two years of life.