Individuals with autism spectrum disorder (ASD) display a pattern of social communication deficits, restricted and repetitive behaviours, and information processing difficulties that leave them particularly vulnerable to developing anxiety. The process of diagnosing anxiety is confounded by the very nature of the communication difficulties found in this population and is further exacerbated when a co-occurring intellectual disability (ID) is also present. As part of this research, a scoping review was conducted into the current research available directly addressing how anxiety is identified or diagnosed in individuals with ASD and ID. This review found very limited research relevant to this population and identified the lack of specifically standardised anxiety measurement tools as a particular issue, as well as the need to consider different symptom criteria in order to more accurately diagnose anxiety in this unique population, with behavioural markers being a useful consideration. For this study, the dynamic model of autism (Bartak et al., 2006) was selected as a theoretical framework that explains how anxiety develops in individuals with ASD without excluding those with a co-occurring ID. Many other theories of anxiety development in autism neglect those with ASD and ID. In this research two key concepts are considered: firstly, the identification and testing of this theoretical framework for understanding how anxiety is related to ASD and ID, and secondly, investigation of behavioural profiles that could be used as markers to identify anxiety in individuals with ASD and ID. Through four hierarchical multiple regressions, the dynamic model of autism was found to offer an explanation for how anxiety develops in individuals with ASD and ID. This theory proposes that it is the very core characteristics of ASD that result in an individual’s vulnerability to developing anxiety, and it may be that those with co-occurring ID are even more at risk given their lack of ability to develop alternative strategies. Bayesian model averaging was used to investigate behavioural patterns predicting anxiety in individuals with ASD and ID. Within this study, several behavioural markers were found to be predictive of increased anxiety, including increased insistence on sameness, increased sensory sensitivity, and increased problem behaviour. By considering that anxiety may be the potential cause of problem behaviours we may in turn have more success with addressing ASD behavioural escalations. Increased recognition of the mental health experiences and the need for appropriate treatment and support for this under-researched population could lead to better quality of life outcomes in general and in the longer term.