Background Falls among adults with intellectual disability (ID) frequently cause physical injury and may negatively impact on quality of life. Studies investigating falls among people with ID have used differing methods and populations, making it difficult to determine the scope and extent of this problem. Objective To synthesize the best available evidence to determine the incidence and prevalence of falls among adults with ID. Inclusion criteria Types of participants This review considered all studies that included adults with ID aged 18 years and older. Condition The current review considered studies which reported percentage/numbers of individuals who fell, and the total number of falls and injurious falls sustained from a fall. Context Studies were included if they were conducted within community or residential settings. Studies that were conducted in hospitals were excluded. Types of studies Cohort studies, case-control and cross-sectional studies were included. Studies that used an experimental design, both randomized controlled and quasi experimental design were also included. Search Strategy Methods A three-step search strategy was undertaken for published and unpublished literature in English from 1990 to 2017. An initial search of MEDLINE and CINAHL was undertaken before a more extensive search was conducted using keywords and index terms across 11 electronic databases. Methodological Quality Two independent reviewers assessed the methodological quality of the included studies using Joanna Briggs Institute standardized critical appraisal instrument for prevalence studies (Joanna Briggs Institute Critical Appraisal Checklist for Studies Reporting Prevalence Data). Data extraction Data was extracted using the standardized extraction tool from the Joanna Briggs Institute's System of the Unified Management, Assessment and Review Information. Data that directly reported or could be used to calculate the incidence and prevalence of falls were extracted. Data synthesis Quantitative data for the number (proportion) of people who fell were pooled in statistical meta-analysis using STATA version 14. Data measuring incidence of falls (rate of falls for the duration of the study) and incidence of injurious falls (rate of falls resulting in one or more injuries for the duration of the study) could not be pooled in meta-analysis, hence results were presented in a narrative form including tables. Standard GRADE (Grading of Recommendations Assessment, Development and Evaluation) evidence assessment of outcomes is also reported. Results Nine studies were eligible for inclusion in this review. Eight articles were observational cohort studies which reported on the incidence/prevalence of falls as outcome measures, one article was a quasi-experimental study design. Overall the methodological quality of the included studies was considered moderate. The pooled proportion of people with ID who fell (4 studies, 854 participants) was 39% [95% CI (0.35%-0.43%), very low GRADE evidence]. The rate of falls (8 studies, 782 partici...