Specialization in language-specific phonotactics is crucial preparation for becoming a native language user. Our finding that infants with large- and medium-sized vocabularies failed to demonstrate learning of phonotactic patterns that violate native language phonotactics suggests that they showed stronger specialization than infants with smaller vocabularies. To further support this point, we are also beginning to examine whether there are vocabulary differences in infants’ processing of native language phonotactics as well. We propose that if infants with small vocabularies have not yet developed specialization for processing native phonotactics, they may have difficulty in exploiting the ways in which phonotactic information supports language acquisition and processing. Knowledge of which consonant clusters can and cannot occur at word onsets and offsets provides cues to word boundaries, facilitating rapid word recognition in adults and facilitating word segmentation and word learning in infants. In addition, knowledge of phonotactic patterns may help to constrain word learning by promoting infants’ focus on the sound sequences that form possible words and allowing them to disregard forms that are not possible or likely to label concepts. Thus, developing robust phonotactic knowledge reflects adaptive language-specific specialization that supports multiple functions in acquisition and processing. Lack of support from phonotactic knowledge may put infants at a disadvantage in vocabulary acquisition.