Prospective longitudinal studies of high-risk infants have provided a nuanced characterisation of behavioural symptom emergence in infancy and toddlerhood. However, our understanding of the causal neurodevelopmental mechanisms remains limited. We present four theoretically motivated approaches designed to provide insight into mechanisms underlying symptom onset. Our first speaker acquires brain structural, functional and biochemical data from fetuses, neonates and infants with and without risk factors for neurodevelopmental conditions. She will present preliminary evidence of differences in regional brain structure and function and maturation of the glutamate system in these risk groups. Second, we showcase recent work demonstrating that patterns of EEG observed in 3-month-old infants at high risk for developing autism and who are subsequently diagnosed with autism at age 3 differ from infants who do not develop autism. Third, we show that alterations in infants’ pupillary light reflex – a basic measure putatively linked to the cholinergic system – may relate to later ASD outcomes. Finally, we will present recent work on altered experience-dependent specialisation of the social brain in early ASD, including new data linking variation to sensory processing atypicalities. Taken together, these talks identify new avenues for generating fundamental insights into the mechanisms that drive symptom onset in ASD.