["Developmental Science, Volume 29, Issue 3, May 2026. ", "\nABSTRACT\n\nInfants with an Elevated Likelihood (EL) for autism are more likely to experience language delays, but their language developmental trajectories are highly heterogeneous. The neurodevelopmental processes driving this variability remain unclear, yet understanding them is important for parsing the heterogeneity in language and informing earlier intervention. However, no studies have examined how developmental changes in brain function are related to changes in language ability across time in this population. In this prospective longitudinal study, we investigated the developmental associations between electroencephalography (EEG) and language measures in a sample of infants enriched for varied language outcomes: Infants with EL and Typical Likelihood (TL) for autism (EL = 99, TL = 60). Measures of EEG power in the relative frontal theta and alpha ranges during natural viewing of social and nonsocial naturalistic videos and observer assessment of language ability were collected at 10, 14, 24 and 36 months. As expected, the EL and TL groups differed in the slope of language ability, with the EL group showing less steep increases in language compared to the TL group. Further, 10‐month theta power (but not alpha) was a significant predictor of developmental change in language; autism likelihood group did not moderate this association and theta development was not related to language. Overall, findings suggest slowed language development as an autism trait symptom and that theta power is a predictor of general language ability, not specific to autism. Future research should consider incorporating additional measures to examine the potential environmental or genetic contributions to these associations.\n\n\nSummary\n\nInfants with Elevated Likelihood (EL) for autism vary in their language development, while the neurodevelopmental processes driving this variability are still unclear.\nThe relation between developmental changes in relative electroencephalography (EEG) spectral power and language ability across 10–36 months in infants with and without an EL for autism was examined.\nThe EL group showed less steep increases in language development compared to the TL group, but the groups did not differ in relative EEG power.\nTheta power was a significant predictor of developmental change in language; this association was not moderated by the autism likelihood group.\n\n\n"]