Research regarding the prevalence of psychiatric conditions co-occurring with intellectual disability in adults was reviewed. Particular attention was paid to the qualities of sampling and diagnostic methodology, which have been identified as needs in two recent reviews. Sixteen articles published in peer-reviewed journals between 2003 and 2009 met inclusion criteria for this review. Overall prevalence rates for co-occurring psychiatric symptoms or disorders reported in these studies ranged from 13.9% to 75.2% with much of this variation due to differences in the diagnostic criteria utilized and the specific samples examined. Results indicated that although several studies have evidenced improvement in methodology, problems remain regarding sampling and general lack of consistency regarding diagnostic definitions and tools. Suggested directions for future research include expansion of geographic and cultural diversity in participants, increased use of population-based sampling, and improved concurrence regarding evaluation methods and diagnostic criteria. (Contains 2 figures.)