Despite several waves of epidemiological studies in adult and elderly mental disorders in the past decades, revealing inconsistent results, little is known about the prevalence of mental disorders in the child and young adolescent population in Taiwan. However, there is an increasing trend of child and adolescent mental disorders over the past decades consistently reported across countries. With the revision of the Diagnostic and Statistical Manual of Mental Disorders to the current version—5th edition (DSM-5), there has been no epidemiological study using DSM-5 in youth population in the world. The DSM-5 was published in October 2014. It warrants an epidemiological study on child and adolescent mental disorders using the DSM-5 as diagnostic criteria in Taiwan.