Background: In the last decades several instruments measuring anxiety in adults with intellectual disabilities have been developed. Aim: To give an overview of the characteristics and psychometric properties of self-report and informant-report instruments measuring anxiety in this group. Method: Systematic review of the literature. Results: Seventeen studies studying 14 different instruments were found. Methodological quality as measured with the Quality Assessment of Diagnostic Accuracy Studies checklist was insufficient for four studies, sufficient for seven, and good for six. For self-report, the Glasgow Anxiety Scale for people with a learning disability appears most promising, with good internal consistency (a = 0.96), high test-retest reliability (r = 0.95), sensitivity (100%) and specificity (100%). For informant-report, the general anxiety subscale of the Anxiety, Depression and Mood Scale may be promising, with good internal consistency (a = 0.83 and a = 0.84) and excellent test-retest reliability (ICC = 0.78 and ICC = 0.92), but poor interrater reliability (ICC = 0.39). Conclusions: Two instruments appear promising. However, these instruments have only been studied once or twice, whereas the methodological quality of these studies was varying.