This research aims to better understand the cognitive and behavioural features of anxiety in individuals with Cornelia de Lange Syndrome, and its overlap with autistic characteristics. Due to overlapping traits, anxiety can be difficult to disentangle from autistic characteristics (Tarver et al., 2021) which can lead to a lack of recognition and support, especially when using clinical assessment tools that have not been designed for use in people with and those with rare genetic syndromes. Chapter two reports a significant association between informant reported anxiety and autistic characteristics, once controlling for self-help skills. Neither were found to be significantly associated with age-related changes. Chapters three and four draw upon two theoretical models of anxiety, both of which model the development and maintenance of anxiety disorders. Chapter three explores fixation patterns towards emotional vs neutral face pairs and socially salient features, finding no initial fixation bias toward emotional vs neutral faces in any of the groups. Findings in the CdLS group showed greater proportional fixation duration and reduced proportional fixation duration to eye region to angry faces compared to NT controls. Autistic characteristics were found to significantly predict fixation duration to eye region for all groups. Chapter four introduced a novel behavioural task to explore behavioural responses to uncertainty. Results showed increased uncertainty-related behaviours as conditions of uncertainty increased, and differences in presentation of uncertainty between groups were approaching significance. Chapter five explores anxiety profiles in CdLS, autistic and NT groups and the cognitive and behavioural correlations associated with anxiety in a subset of CdLS participants. Findings revealed greater anxiety in CdLS and autistic groups particularly on the uncertainty subscale of the ASC-ASD for individuals with CdLS. These findings lead to an improved understanding of anxiety in CdLS and encourage the development of more targeted, syndrome sensitive early intervention strategies.