PROJECT SUMMARY/ABSTRACT The goal of this research is to examine early bimanual motor skill as a predictor for language development. Motor experience provides rich opportunities for learning in infants. Fine motor skills cascade from initial success reaching and grasping an object to more complex exploration where one hand holds the object for the other hand's manipulation (i.e., role-differentiated bimanual manipulation, or RDBM). A consequence of new motor skills like RDBM is a dramatic shift in how infants engage in their social world, and infants and toddlers who demonstrate proficiency in a motor skill are more likely to exhibit advanced language skills. The majority of work on motor-language cascades has examined postural or locomotor skills. The developmental interplay between early manual skill and later language ability is not well understood. Hand use patterns in infants matter – infants who exhibit a consistent hand use trajectory for reaching to objects show advanced language skills at 2 years of age compared to their inconsistent counterparts. As toddlers, nearly all children exhibit a stable hand use pattern for RDBM, and consistency in hand use for RDBM continues to predict language outcomes at 3 years of age. However, there is a gap in our knowledge of hand use patterning during the period when RDBM emerges towards the end of the first year of life. During this period of variability, the development of RDBM may be characterized by different hand use trajectories. Secondary analyses will be conducted on data from a longitudinal study on hand use that videotaped children engaging with objects that afford RDBM as infants, and assessed language ability at three preschool follow up visits. The aims of the project are to characterize developmental trajectories of RDBM proficiency using latent class analysis and to use RDBM trajectories to predict distal language outcomes. Infants who exhibit greater proficiency in RDBM are predicted to have advanced language skills as preschoolers. Indexing RDBM proficiency and characterizing motor- language cascades has important implications for developing benchmarks for typically developing children as well as children with or at risk for developmental delays and disorders.