This article reviews the prevalence of mental disorders in children and adolescents based on recent community surveys worldwide. Despite methodological variations, findings consistently show that about one-fourth of youth experience a mental disorder in the past year, and about one-third across their lifetimes. Anxiety disorders are the most common, followed by behavior disorders, mood disorders, and substance use disorders. Fewer than half of youth with mental disorders receive specialized mental health treatment, though those with severe disorders are more likely to receive services. Key issues in child psychiatric epidemiology include refining classification and assessment, including young children in surveys, integrating child and adult studies, and evaluating both mental and physical disorders in children. The World Health Organization states that mental health disorders are a leading cause of disability globally, with three of the top ten causes of disability in 15-44-year-olds being mental disorders. Most adult mental disorders begin in childhood or adolescence, highlighting the importance of understanding the magnitude, risk factors, and progression of mental disorders in youth. The aims of this review are to provide background on epidemiology, summarize prevalence estimates of specific mental disorders in children, describe correlates and risk factors, and outline key issues and future directions in child mental disorder research. Epidemiology studies the distribution and determinants of diseases in populations, aiming to identify disease causes to prevent or intervene. Descriptive epidemiology specifies disorder rates and distribution, while analytic epidemiology focuses on disease determinants. Prevalence and incidence rates are adjusted for gender and age. Epidemiologic studies identify risk factors influencing disease rates, including gender, age, ethnicity, and exposure to risk factors. Case-control studies compare risk factors with disease presence, controlling for confounding variables. Community study data help identify biases in treated populations and construct case registries for epidemiologic studies. Sampling issues are crucial as clinical settings may not represent the general population. Epidemiology tools have led to methodological developments and substantive findings about mental disorder prevalence, comorbidity, and service patterns. Future research should focus on how multiple risk factors interact over time. Prevalence of mental disorders in youth is high, with about one-third meeting lifetime criteria for a DSM disorder. Only a small proportion of these youth have severe distress warranting intervention. About one in ten youths meet criteria for a Serious Emotional Disturbance. Depressive disorders have varying prevalence rates, with higher rates in females during adolescence. Longitudinal studies show an average age of onset between 11 and 14 years for MDD and depressive disorders. Incidence increases through early adulthood, with no significant gender differences in onset age. Bipolar disorder has low prevalence in children, with incidence peaking at age 14. Anxiety disorders are the most prevalent in children, with girls more likely to have various subtypes. Anxiety disorders often co-occur with other mental disorders, suggesting a developmental sequence. ADHD has varying prevalence rates, with higher rates in boys. RiskThis article reviews the prevalence of mental disorders in children and adolescents based on recent community surveys worldwide. Despite methodological variations, findings consistently show that about one-fourth of youth experience a mental disorder in the past year, and about one-third across their lifetimes. Anxiety disorders are the most common, followed by behavior disorders, mood disorders, and substance use disorders. Fewer than half of youth with mental disorders receive specialized mental health treatment, though those with severe disorders are more likely to receive services. Key issues in child psychiatric epidemiology include refining classification and assessment, including young children in surveys, integrating child and adult studies, and evaluating both mental and physical disorders in children. The World Health Organization states that mental health disorders are a leading cause of disability globally, with three of the top ten causes of disability in 15-44-year-olds being mental disorders. Most adult mental disorders begin in childhood or adolescence, highlighting the importance of understanding the magnitude, risk factors, and progression of mental disorders in youth. The aims of this review are to provide background on epidemiology, summarize prevalence estimates of specific mental disorders in children, describe correlates and risk factors, and outline key issues and future directions in child mental disorder research. Epidemiology studies the distribution and determinants of diseases in populations, aiming to identify disease causes to prevent or intervene. Descriptive epidemiology specifies disorder rates and distribution, while analytic epidemiology focuses on disease determinants. Prevalence and incidence rates are adjusted for gender and age. Epidemiologic studies identify risk factors influencing disease rates, including gender, age, ethnicity, and exposure to risk factors. Case-control studies compare risk factors with disease presence, controlling for confounding variables. Community study data help identify biases in treated populations and construct case registries for epidemiologic studies. Sampling issues are crucial as clinical settings may not represent the general population. Epidemiology tools have led to methodological developments and substantive findings about mental disorder prevalence, comorbidity, and service patterns. Future research should focus on how multiple risk factors interact over time. Prevalence of mental disorders in youth is high, with about one-third meeting lifetime criteria for a DSM disorder. Only a small proportion of these youth have severe distress warranting intervention. About one in ten youths meet criteria for a Serious Emotional Disturbance. Depressive disorders have varying prevalence rates, with higher rates in females during adolescence. Longitudinal studies show an average age of onset between 11 and 14 years for MDD and depressive disorders. Incidence increases through early adulthood, with no significant gender differences in onset age. Bipolar disorder has low prevalence in children, with incidence peaking at age 14. Anxiety disorders are the most prevalent in children, with girls more likely to have various subtypes. Anxiety disorders often co-occur with other mental disorders, suggesting a developmental sequence. ADHD has varying prevalence rates, with higher rates in boys. Risk