This chapter reviews important methodological considerations relevant to comparing and contrasting estimated rates of mental illness among homeless populations in the US and nations in Europe. Existing research on international prevalence estimates is presented to highlight the impact of definitional and methodological differences on obtained rates of mental illness among the homeless. It is suggested that variation in estimates across nations observed in existing studies is due primarily to varying operationalizations of homelessness, sampling methodology, and mental health measures. The chapter also describes recommendations for future research on this topic.