Background: Beliefs about the controllability of behaviour have been consistently shown to be important in understanding the responses of carers to the challenging behaviour of people with intellectual disabilities (IDs). This paper reports the reliability and validity of the Controllability Beliefs Scale (CBS), a 15-item measure of beliefs regarding the controllability of challenging behaviour when used with carers of people with IDs. Methods: Two hundred and sixty-four carers of people with IDs completed the CBS, 74 people also completed the Modified Attributional Style Questionnaire and the Self-Injury Behavioural Understanding Questionnaire scale to determine concurrent and convergent validity and 34 people completed the scale twice within a 2- to 4-week period to determine test-retest reliability. Results: The scale has a two-factor structure and has adequate internal reliable. The scale is significantly correlated with the controllability, internality and stability items from the Modified Attributional Style Questionnaire, showed expected associations with behavioural and internal emotional understanding items from the Self-Injury Behavioural Understanding Questionnaire. The scale has good test-retest reliability. Conclusions: The data support use of the CBS in clinical practice and research to assess carers' beliefs regarding challenging behaviour of people with IDs. (Contains 2 tables.)