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Autism Spectrum Disorder (ASD) is a heritable neurodevelopmental condition that is characterised by social communication impairments, restricted and repetitive behaviours, and sensory anomalies. Anxiety is one of the marked co-occurring psychiatric conditions in individuals with ASD and the underlying mechanism of this co-occurrence has not been fully understood. This is because studies have focused on mid-childhood or adolescence when the interplay between genetic and environmental risk factors make it harder to disentangle the overlap between symptoms of anxiety and ASD. Temperament traits in infancy, especially behavioural inhibition (BI) which is a temperament trait that is involved in the aetiology of the childhood anxiety in the general population, may be an informative target to explore roots of this interplay prior to the consolidation of both disorders. The current thesis employs a multi-method approach to investigate the association between BI and anxiety in two cohorts of infants at high- and low-familial risk for ASD. Participants of Chapter 2 and 3 were drawn from the second phase of the British Autism Study of Infant Siblings (BASIS; this study is complete and outcome grouping is available). In Chapter 2, longitudinal associations between parent-reported BI, effortful control, anxiety and ASD traits were examined using cross-lagged panel models. In Chapter 3, temperament traits were measured during the Autism Observational Schedule for Infants (AOSI; 15 months) by using a new observational coding scheme. Further analyses investigated whether observed individual differences relate to AOSI scores, anxiety and ASD traits at 36 months. In Chapter 4, factor scores for social and non-social BI were generated at 24 months by using observational, parent-reported and global ratings of BI. In Chapter 5, the associations between parental and child anxiety and ASD trait was examined. Participants of Chapter 4 and 5 were drawn from the third phase of the BASIS project. Both parent-reported and observed BI was associated with higher levels of anxiety but not ASD traits. Parental characteristics (anxiety and ASD traits) were related to child characteristics. Overall, these findings suggest that similar to the general population, BI is involved in the aetiology of early emerging anxiety traits in toddlers at risk of ASD. The consistent association between BI and anxiety across chapters suggest that there may be separate developmental pathways for anxiety and ASD. So, BI may provide a translational target for pre-emptive interventions.