An important issue in assessing the societal burden of mental disorders is whether the evidence of increasing prevalence in recent cohorts is real or a methodological artifact. The chapter begins with a broad overview of results concerning the estimated lifetime prevalence, age-of-onset distributions, projected lifetime risk, cohort effects, and sociodemographic correlates of the Diagnostic and Statistical Manual DSM-IV disorders assessed in the National Comorbidity Survey Replication (NCS-R). It then turns to a discussion of the prevalence of these same disorders in the year before the NCS-R interview. This is followed by a brief review of data regarding trends in disorder prevalence and treatment in the NCS-R compared to a decade earlier in the baseline NCS. The chapter closes with a discussion of interpretations and implications of these results along with anticipated future directions in the investigation of the prevalence of mental disorders.