There is a general consensus that, for several reasons, people with mental retardation are at an increased risk of developing emotional disorders. Numerous research studies have examined the prevalence of psychiatric disorders among people with mental retardation, and a wide range of rates have been reported. Reasons for the variability in these results are discussed, including definitional and identification issues, and sampling issues. A summary of representative studies is presented and discussed in light of the above-mentioned issues. The need for updated epidemiological studies in this area is emphasized.