To understand language, children must listen for meaning in speech sounds that unfold rapidly in time. Researchers have begun to explore the emergence of early efficiency in language understanding in typically developing (TD) infants, but little is known about how speech processing abilities develop in children who are at risk for language delay (LD), the question that motivates this proposal. Two longitudinal studies with English- and Spanish-learning children at risk for language delay explore relations between online measures of processing efficiency and traditional measures of language and cognitive competence on experimental and standardized tests. Participants in Study I, (n=125) recruited at 18 months, include English-learning TD children as well as late talkers (LT), some who will later move into the normal range, and some who will remain at risk for language delay. These children are tested in the looking-while-listening (LWL) procedure to assess speech processing skill at 18, 24, 30, 36, & 48 months. Participants in Study II (n=126) are Spanish-learning children at risk for language delay, as well as bilingual children learning Spanish and English simultaneously, also recruited at 18 months including TD and LT children. These Latino children are assessed at 18, 24, 36, 48, & 60 months. One goal is to adapt online processing measures developed with TD children to focus on Language Processing Challenges (LPCs) in four critical areas of language competence: familiar word recognition; use of morphosyntactic information; semantic integration; and novel word learning, all areas in which LD children have difficulties. A second goal is to establish the validity of these new online processing measures for use with very young children in relation to traditional measures of lexical and grammatical growth and to standardized clinical instruments used to assess language delay. At each age children are tested in the online LWL procedure, yielding precise, sensitive, continuous measures of speed and accuracy in interpreting linguistic stimuli in real time. They are also assessed at each age on standardized tests, enabling us to examine concurrent relations between processing efficiency in each LPC and more traditional measures of language development. Our overarching goal is to determine which measures of language function are most powerful in predicting later outcomes in diverse populations. We use online measures to identify which particular processing problems may be early precursors or markers of later language impairment. Our longitudinal designs prospectively track growth in language skills, allowing us to determine to what extent processing efficiency in infancy can distinguish LT children who catch up from those who are at risk for language and learning difficulties. Although our major focus is on the clinical relevance of these measures for children at risk for language delay, Studies I & II will also enable us to explore the full continuum of receptive language skill in diverse populations of children learning Spanish and English, documenting individual differences in children who are highly competent as well as those who are having difficulty in spoken language processing.