Results of existing literature suggest that the relationship between acculturation and mental health is complex. Some studies have found a beneficial association between increased acculturation and mental health, whereas others have found a detrimental association or no relationship at all. We reviewed literature on acculturation and mental health in Hispanics/Latino Americans, Asians/Asian Americans, and other ethnic groups. Results indicate that greater acculturation is associated with increased substance use and abuse. Findings for other disorders and symptoms demonstrate considerable heterogeneity and potential reasons for this variation are provided. Acculturation involves a complex set of processes that appear to have differential impact on mental health outcomes. Critical issues in the measurement of acculturation are discussed, and recommendations for future research are offered.