Challenging behaviour displayed by people with intellectual disabilities (ID) can be difficult to manage if caregivers do not understand the reasons for the behaviour. Identifying the contextual variables/triggers for the behaviour is likely to help undertake a functional analysis leading to a person-centred positive behaviour support plan. Currently, a limited number of checklists are available for trigger assessment and none were developed using an interview with the family caregivers. This article describes the development and contents of the comprehensive assessment of triggers for behaviours of concern scale (CATS). CATS was developed in two stages. Stage 1 used a ‘bottom-up’ approach, in which caregivers of adults with ID who show aggressive behaviour were interviewed to identify the triggers for aggression. In stage two, using a ‘top-down’ approach, a comprehensive literature review was conducted to gather items from existing trigger checklists. Trigger items from both stages were combined and the duplicates were removed. The final list in CATS consists of 333 contextual triggers categorised under five main domains and 12 subdomains. CATS can be used by caregivers to identify triggers or antecedents of challenging behaviour. Further work is needed to test its psychometric properties, utility, and acceptability.